feat: #128 #208 #42 #55

39.combination-sum.js

+/*
+ * @lc app=leetcode id=39 lang=javascript
+ *
+ * [39] Combination Sum
+ *
+ * https://leetcode.com/problems/combination-sum/description/
+ *
+ * algorithms
+ * Medium (46.89%)
+ * Total Accepted:    326.7K
+ * Total Submissions: 684.2K
+ * Testcase Example:  '[2,3,6,7]\n7'
+ *
+ * Given a set of candidate numbers (candidates) (without duplicates) and a
+ * target number (target), find all unique combinations in candidates where the
+ * candidate numbers sums to target.
+ *
+ * The same repeated number may be chosen from candidates unlimited number of
+ * times.
+ *
+ * Note:
+ *
+ *
+ * All numbers (including target) will be positive integers.
+ * The solution set must not contain duplicate combinations.
+ *
+ *
+ * Example 1:
+ *
+ *
+ * Input: candidates = [2,3,6,7], target = 7,
+ * A solution set is:
+ * [
+ * ⁠ [7],
+ * ⁠ [2,2,3]
+ * ]
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: candidates = [2,3,5], target = 8,
+ * A solution set is:
+ * [
+ * [2,2,2,2],
+ * [2,3,3],
+ * [3,5]
+ * ]
+ *
+ */
+
+function backtrack(list, tempList, nums, remain, start) {
+  if (remain < 0) return;
+  else if (remain === 0) return list.push([...tempList]);
+  for (let i = start; i < nums.length; i++) {
+    tempList.push(nums[i]);
+    backtrack(list, tempList, nums, remain - nums[i], i); // 数字可以重复使用， i + 1代表不可以重复利用
+    tempList.pop();
+  }
+}
+/**
+ * @param {number[]} candidates
+ * @param {number} target
+ * @return {number[][]}
+ */
+var combinationSum = function(candidates, target) {
+  const list = [];
+  backtrack(list, [], candidates.sort((a, b) => a - b), target, 0);
+  return list;
+};

40.combination-sum-ii.js

+/*
+ * @lc app=leetcode id=40 lang=javascript
+ *
+ * [40] Combination Sum II
+ *
+ * https://leetcode.com/problems/combination-sum-ii/description/
+ *
+ * algorithms
+ * Medium (40.31%)
+ * Total Accepted:    212.8K
+ * Total Submissions: 519K
+ * Testcase Example:  '[10,1,2,7,6,1,5]\n8'
+ *
+ * Given a collection of candidate numbers (candidates) and a target number
+ * (target), find all unique combinations in candidates where the candidate
+ * numbers sums to target.
+ * 
+ * Each number in candidates may only be used once in the combination.
+ * 
+ * Note:
+ * 
+ * 
+ * All numbers (including target) will be positive integers.
+ * The solution set must not contain duplicate combinations.
+ * 
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: candidates = [10,1,2,7,6,1,5], target = 8,
+ * A solution set is:
+ * [
+ * ⁠ [1, 7],
+ * ⁠ [1, 2, 5],
+ * ⁠ [2, 6],
+ * ⁠ [1, 1, 6]
+ * ]
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: candidates = [2,5,2,1,2], target = 5,
+ * A solution set is:
+ * [
+ * [1,2,2],
+ * [5]
+ * ]
+ * 
+ * 
+ */
+function backtrack(list, tempList, nums, remain, start) {
+    if (remain < 0) return;
+    else if (remain === 0) return list.push([...tempList]);
+    for (let i = start; i < nums.length; i++) {
+      if(i > start && nums[i] == nums[i-1]) continue; // skip duplicates
+      tempList.push(nums[i]);
+      backtrack(list, tempList, nums, remain - nums[i], i + 1); // i + 1代表不可以重复利用， i 代表数字可以重复使用 
+      tempList.pop();
+    }
+  }
+/**
+ * @param {number[]} candidates
+ * @param {number} target
+ * @return {number[][]}
+ */
+var combinationSum2 = function(candidates, target) {
+    const list = [];
+    backtrack(list, [], candidates.sort((a, b) => a - b), target, 0);
+    return list;
+};

46.permutations.js

+/*
+ * @lc app=leetcode id=46 lang=javascript
+ *
+ * [46] Permutations
+ *
+ * https://leetcode.com/problems/permutations/description/
+ *
+ * algorithms
+ * Medium (53.60%)
+ * Total Accepted:    344.6K
+ * Total Submissions: 642.9K
+ * Testcase Example:  '[1,2,3]'
+ *
+ * Given a collection of distinct integers, return all possible permutations.
+ * 
+ * Example:
+ * 
+ * 
+ * Input: [1,2,3]
+ * Output:
+ * [
+ * ⁠ [1,2,3],
+ * ⁠ [1,3,2],
+ * ⁠ [2,1,3],
+ * ⁠ [2,3,1],
+ * ⁠ [3,1,2],
+ * ⁠ [3,2,1]
+ * ]
+ * 
+ * 
+ */
+function backtrack(list, tempList, nums) {
+    if (tempList.length === nums.length) return list.push([...tempList]);
+    for(let i = 0; i < nums.length; i++) {
+        if (tempList.includes(nums[i])) continue;
+        tempList.push(nums[i]);
+        backtrack(list, tempList, nums);
+        tempList.pop();
+    }
+}
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var permute = function(nums) {
+    const list = [];
+    backtrack(list, [], nums)
+    return list
+};
+

47.permutations-ii.js

+/*
+ * @lc app=leetcode id=47 lang=javascript
+ *
+ * [47] Permutations II
+ *
+ * https://leetcode.com/problems/permutations-ii/description/
+ *
+ * algorithms
+ * Medium (39.29%)
+ * Total Accepted:    234.1K
+ * Total Submissions: 586.2K
+ * Testcase Example:  '[1,1,2]'
+ *
+ * Given a collection of numbers that might contain duplicates, return all
+ * possible unique permutations.
+ *
+ * Example:
+ *
+ *
+ * Input: [1,1,2]
+ * Output:
+ * [
+ * ⁠ [1,1,2],
+ * ⁠ [1,2,1],
+ * ⁠ [2,1,1]
+ * ]
+ *
+ *
+ */
+function backtrack(list, nums, tempList, visited) {
+  if (tempList.length === nums.length) return list.push([...tempList]);
+  for (let i = 0; i < nums.length; i++) {
+    if (visited[i]) continue;
+    // visited[i - 1] 容易忽略
+    if (i > 0 && nums[i] === nums[i - 1] && visited[i - 1]) continue;
+
+    visited[i] = true;
+    tempList.push(nums[i]);
+    backtrack(list, nums, tempList, visited);
+    visited[i] = false;
+    tempList.pop();
+  }
+}
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var permuteUnique = function(nums) {
+  const list = [];
+  backtrack(list, nums.sort((a, b) => a - b), [], []);
+  return list;
+};

62.unique-paths.js

+/*
+ * @lc app=leetcode id=62 lang=javascript
+ *
+ * [62] Unique Paths
+ *
+ * https://leetcode.com/problems/unique-paths/description/
+ *
+ * algorithms
+ * Medium (46.53%)
+ * Total Accepted:    277K
+ * Total Submissions: 587.7K
+ * Testcase Example:  '3\n2'
+ *
+ * A robot is located at the top-left corner of a m x n grid (marked 'Start' in
+ * the diagram below).
+ *
+ * The robot can only move either down or right at any point in time. The robot
+ * is trying to reach the bottom-right corner of the grid (marked 'Finish' in
+ * the diagram below).
+ *
+ * How many possible unique paths are there?
+ *
+ *
+ * Above is a 7 x 3 grid. How many possible unique paths are there?
+ *
+ * Note: m and n will be at most 100.
+ *
+ * Example 1:
+ *
+ *
+ * Input: m = 3, n = 2
+ * Output: 3
+ * Explanation:
+ * From the top-left corner, there are a total of 3 ways to reach the
+ * bottom-right corner:
+ * 1. Right -> Right -> Down
+ * 2. Right -> Down -> Right
+ * 3. Down -> Right -> Right
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: m = 7, n = 3
+ * Output: 28
+ *
+ *   START
+ */
+/**
+ * @param {number} m
+ * @param {number} n
+ * @return {number}
+ */
+var uniquePaths = function(m, n) {
+  // backtracking
+  const dp = [];
+  for (let i = 0; i < m + 1; i++) {
+    dp[i] = [];
+    dp[i][0] = 0;
+  }
+  for (let i = 0; i < n + 1; i++) {
+    dp[0][i] = 0;
+  }
+  for (let i = 1; i < m + 1; i++) {
+    for(let j = 1; j < n + 1; j++) {
+        dp[i][j] = j === 1 ? 1 : dp[i - 1][j] + dp[i][j - 1];
+    }
+  }
+
+  return dp[m][n];
+};

78.subsets.js

+/*
+ * @lc app=leetcode id=78 lang=javascript
+ *
+ * [78] Subsets
+ *
+ * https://leetcode.com/problems/subsets/description/
+ *
+ * algorithms
+ * Medium (51.19%)
+ * Total Accepted:    351.6K
+ * Total Submissions: 674.8K
+ * Testcase Example:  '[1,2,3]'
+ *
+ * Given a set of distinct integers, nums, return all possible subsets (the
+ * power set).
+ * 
+ * Note: The solution set must not contain duplicate subsets.
+ * 
+ * Example:
+ * 
+ * 
+ * Input: nums = [1,2,3]
+ * Output:
+ * [
+ * ⁠ [3],
+ * [1],
+ * [2],
+ * [1,2,3],
+ * [1,3],
+ * [2,3],
+ * [1,2],
+ * []
+ * ]
+ * 
+ */
+function backtrack(list, tempList, nums, start) {
+    list.push([...tempList]);
+    for(let i = start; i < nums.length; i++) {
+        tempList.push(nums[i]);
+        backtrack(list, tempList, nums, i + 1);
+        tempList.pop();
+    }
+}
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var subsets = function(nums) {
+    const list = [];
+    backtrack(list, [], nums, 0);
+    return list;
+};
+

90.subsets-ii.js

+/*
+ * @lc app=leetcode id=90 lang=javascript
+ *
+ * [90] Subsets II
+ *
+ * https://leetcode.com/problems/subsets-ii/description/
+ *
+ * algorithms
+ * Medium (41.53%)
+ * Total Accepted:    197.1K
+ * Total Submissions: 469.1K
+ * Testcase Example:  '[1,2,2]'
+ *
+ * Given a collection of integers that might contain duplicates, nums, return
+ * all possible subsets (the power set).
+ * 
+ * Note: The solution set must not contain duplicate subsets.
+ * 
+ * Example:
+ * 
+ * 
+ * Input: [1,2,2]
+ * Output:
+ * [
+ * ⁠ [2],
+ * ⁠ [1],
+ * ⁠ [1,2,2],
+ * ⁠ [2,2],
+ * ⁠ [1,2],
+ * ⁠ []
+ * ]
+ * 
+ * 
+ */
+function backtrack(list, tempList, nums, start) {
+    list.push([...tempList]);
+    for(let i = start; i < nums.length; i++) {
+        if (i > start && nums[i] === nums[i - 1]) continue;
+        tempList.push(nums[i]);
+        backtrack(list, tempList, nums, i + 1)
+        tempList.pop();
+    }
+}
+/**
+ * @param {number[]} nums
+ * @return {number[][]}
+ */
+var subsetsWithDup = function(nums) {
+    const list = [];
+    backtrack(list, [], nums.sort((a, b) => a - b), 0, [])
+    return list;
+};
+

README.md

@@ -53,9 +53,9 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 ## 精彩预告
 
 
-301. remove-invalid-parentheses:
+42.trapping-rain-water-1(雨水收集问题):
 
-![remove-invalid-parentheses](./assets/problems/301.remove-invalid-parentheses.png)
+![42.trapping-rain-water](./assets/problems/42.trapping-rain-water-1.png)
 
 浏览器中的栈：
 
@@ -98,6 +98,7 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 - [11.container-with-most-water](./problems/11.container-with-most-water.md)
 - [19. Remove Nth Node From End of List](./problems/removeNthNodeFromEndofList.md)
 - [24. Swap Nodes In Pairs](./problems/swapNodesInPairs.md)
+- [55.jump-game.md](./problems/55.jump-game.md.md)
 - [75.sort-colors.md](./problems/75.sort-colors.md)
 - [86.partition-list](./problems/86.partition-list.md)
 - [92.reverse-linked-list-ii](./problems/92.reverse-linked-list-ii.md)
@@ -109,6 +110,7 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 - [150.evaluate-reverse-polish-notation](./problems/150.evaluate-reverse-polish-notation.md)
 - [199.binary-tree-right-side-view](./problems/199.binary-tree-right-side-view.md)
 - [201.bitwise-and-of-numbers-range](./problems/201.bitwise-and-of-numbers-range.md)
+- [208.implement-trie-prefix-tree](./problems/208.implement-trie-prefix-tree.md)
 - [209.minimum-size-subarray-sum](./problems/209.minimum-size-subarray-sum.md)
 - 🆕 [240.search-a-2-d-matrix-ii](./problems/240.search-a-2-d-matrix-ii.md)
 - [279.perfect-squares](./problems/279.perfect-squares.md)
@@ -123,7 +125,8 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 - [900.rle-iterator](./problems/900.rle-iterator.md)
 
 #### 困难难度
-
+- [42.trapping-rain-water](./problems/42.trapping-rain-water.md)
+- [128.longest-consecutive-sequence](./problems/128.longest-consecutive-sequence.md)
 - [145.binary-tree-postorder-traversal](./problems/145.binary-tree-postorder-traversal.md)
 - [146.lru-cache](./problems/146.lru-cache.md)
 - 🆕 [295.find-median-from-data-stream.md](./problems/295.find-median-from-data-stream.md)
@@ -151,9 +154,13 @@ anki - 文件 - 导入 - 下拉格式选择“打包的 anki集合”，然后
 
 ### 计划
 
-- [494.target-sum]
+- [494.target-sum](./todo/494.target-sum.js)
+
+- [609.find-duplicate-file-in-system](./todo/609.find-duplicate-file-in-system.js)
+
+- [10.regular-expression-matching](./todo/10.regular-expression-matching.js)
 
-- [609.find-duplicate-file-in-system]
+- [365.water-and-jug-problem](./todo/365.water-and-jug-problem.js)
 
 - anki 卡片 完善
 
@@ -165,6 +172,6 @@ anki - 文件 - 导入 - 下拉格式选择“打包的 anki集合”，然后
 
 (qq 群)
 
-![wechat-group-chat](./assets/wechat-group-chat.png)
+![wechat-group-chat](./assets/wechat-group-chat.jpg)
 
 (微信群， 由于微信的限制，只可以七天之内才能加入)

problems/128.longest-consecutive-sequence.md

+## 题目地址
+
+https://leetcode.com/problems/longest-consecutive-sequence/description/
+
+## 题目描述
+
+```
+Given an unsorted array of integers, find the length of the longest consecutive elements sequence.
+
+Your algorithm should run in O(n) complexity.
+
+Example:
+
+Input: [100, 4, 200, 1, 3, 2]
+Output: 4
+Explanation: The longest consecutive elements sequence is [1, 2, 3, 4]. Therefore its length is 4.
+Accepted
+200,786
+Submissions
+485,346
+
+```
+
+## 思路
+
+这是一道最最长连续数字序列长度的题目， 官网给出的难度是`hard`.
+
+符合直觉的做法是先排序，然后用一个变量记录最大值，遍历去更新最大值即可，
+
+代码：
+
+```js
+if (nums.length === 0) return 0;
+let count = 1;
+let maxCount = 1;
+// 这里其实可以不需要排序，这么做只不过是为了方便理解
+nums = [...new Set(nums)].sort((a, b) => a - b);
+for (let i = 0; i < nums.length - 1; i++) {
+  if (nums[i + 1] - nums[i] === 1) {
+    count++;
+  } else {
+    if (count > maxCount) {
+      maxCount = count;
+    }
+    count = 1;
+  }
+}
+return Math.max(count, maxCount);
+```
+
+但是需要排序时间复杂度会上升，题目要求时间复杂度为 O(n),
+那么我们其实可以不用排序去解决的。
+
+思路就是将之前”排序之后，通过比较前后元素是否相差 1 来判断是否连续“的思路改为
+不排序而是`直接遍历，然后在内部循环里面查找是否存在当前值的邻居元素`，但是马上有一个
+问题，内部我们`查找是否存在当前值的邻居元素`的过程如果使用数组时间复杂度是 O(n),
+那么总体的复杂度就是 O(n^2)，完全不可以接受。怎么办呢？
+
+我们换个思路，用空间来换时间。比如用类似于 hashmap 这样的数据结构优化查询部分，将时间复杂度降低到 O(1), 代码见后面`代码部分`
+
+## 关键点解析
+
+- 空间换时间
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=128 lang=javascript
+ *
+ * [128] Longest Consecutive Sequence
+ *
+ * https://leetcode.com/problems/longest-consecutive-sequence/description/
+ *
+ * algorithms
+ * Hard (40.98%)
+ * Total Accepted:    200.3K
+ * Total Submissions: 484.5K
+ * Testcase Example:  '[100,4,200,1,3,2]'
+ *
+ * Given an unsorted array of integers, find the length of the longest
+ * consecutive elements sequence.
+ *
+ * Your algorithm should run in O(n) complexity.
+ *
+ * Example:
+ *
+ *
+ * Input: [100, 4, 200, 1, 3, 2]
+ * Output: 4
+ * Explanation: The longest consecutive elements sequence is [1, 2, 3, 4].
+ * Therefore its length is 4.
+ *
+ *
+ */
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var longestConsecutive = function(nums) {
+  nums = new Set(nums);
+  let max = 0;
+  let y = 0;
+  nums.forEach(x => {
+    if (!nums.has(x - 1)) {
+      y = x + 1;
+      while (nums.has(y)) {
+        y = y + 1;
+      }
+      max = Math.max(max, y - x); // y - x 就是从i开始到最后有多少连续的数字
+    }
+  });
+  return max;
+};
+```

problems/208.implement-trie-prefix-tree.md

+## 题目地址
+
+https://leetcode.com/problems/implement-trie-prefix-tree/description/
+
+## 题目描述
+
+```
+Implement a trie with insert, search, and startsWith methods.
+
+Example:
+
+Trie trie = new Trie();
+
+trie.insert("apple");
+trie.search("apple");   // returns true
+trie.search("app");     // returns false
+trie.startsWith("app"); // returns true
+trie.insert("app");
+trie.search("app");     // returns true
+Note:
+
+You may assume that all inputs are consist of lowercase letters a-z.
+All inputs are guaranteed to be non-empty strings.
+
+```
+
+## 思路
+
+这是一道很直接的题目，上来就让你实现`前缀树（字典树）`。这算是基础数据结构中的
+知识了，不清楚什么是字典树的可以查阅相关资料。
+
+我们看到题目给出的使用方法`new Trie`, `insert`,`search`和`startWith`.
+
+为了区分`search`和`startWith`我们需要增加一个标示来区分当前节点是否是某个单词的结尾。
+因此节点的数据结构应该是:
+
+```js
+function TrieNode(val) {
+  this.val = val; // 当前的字母
+  this.children = []; // 题目要求字典仅有a-z，那么其长度最大为26（26个字母）
+  this.isWord = false;
+}
+```
+
+每次 insert 我们其实都是从根节点出发，一个一个找到我们需要添加的节点，修改 children 的值.
+
+我们应该修改哪一个 child 呢？ 我们需要一个函数来计算索引
+
+```js
+function computeIndex(c) {
+  return c.charCodeAt(0) - "a".charCodeAt(0);
+}
+```
+
+其实不管 insert， search 和 startWith 的逻辑都是差不多的，都是从 root 出发，
+找到我们需要操作的 child， 然后进行相应操作（添加，修改，返回）。
+
+![208.implement-trie-prefix-tree-1](../assets/problems/208.implement-trie-prefix-tree-1.png)
+
+## 关键点解析
+
+- 前缀树
+
+- 核心逻辑
+
+```js
+ const c = word[i];
+ const current = computeIndex(c)
+if (!ws.children[current]) {
+    ws.children[current] = new TrieNode(c);
+  }
+  ws = ws.children[current]; // 深度递增
+}
+
+```
+
+## 代码
+
+```js
+/*
+ * @lc app=leetcode id=208 lang=javascript
+ *
+ * [208] Implement Trie (Prefix Tree)
+ *
+ * https://leetcode.com/problems/implement-trie-prefix-tree/description/
+ *
+ * algorithms
+ * Medium (36.93%)
+ * Total Accepted:    172K
+ * Total Submissions: 455.5K
+ * Testcase Example:  '["Trie","insert","search","search","startsWith","insert","search"]\n[[],["apple"],["apple"],["app"],["app"],["app"],["app"]]'
+ *
+ * Implement a trie with insert, search, and startsWith methods.
+ *
+ * Example:
+ *
+ *
+ * Trie trie = new Trie();
+ *
+ * trie.insert("apple");
+ * trie.search("apple");   // returns true
+ * trie.search("app");     // returns false
+ * trie.startsWith("app"); // returns true
+ * trie.insert("app");
+ * trie.search("app");     // returns true
+ *
+ *
+ * Note:
+ *
+ *
+ * You may assume that all inputs are consist of lowercase letters a-z.
+ * All inputs are guaranteed to be non-empty strings.
+ *
+ *
+ */
+function TrieNode(val) {
+  this.val = val;
+  this.children = [];
+  this.isWord = false;
+}
+
+function computeIndex(c) {
+  return c.charCodeAt(0) - "a".charCodeAt(0);
+}
+/**
+ * Initialize your data structure here.
+ */
+var Trie = function() {
+  this.root = new TrieNode(null);
+};
+
+/**
+ * Inserts a word into the trie.
+ * @param {string} word
+ * @return {void}
+ */
+Trie.prototype.insert = function(word) {
+  let ws = this.root;
+  for (let i = 0; i < word.length; i++) {
+    const c = word[i];
+    const current = computeIndex(c);
+    if (!ws.children[current]) {
+      ws.children[current] = new TrieNode(c);
+    }
+    ws = ws.children[current];
+  }
+  ws.isWord = true;
+};
+
+/**
+ * Returns if the word is in the trie.
+ * @param {string} word
+ * @return {boolean}
+ */
+Trie.prototype.search = function(word) {
+  let ws = this.root;
+  for (let i = 0; i < word.length; i++) {
+    const c = word[i];
+    const current = computeIndex(c);
+    if (!ws.children[current]) return false;
+    ws = ws.children[current];
+  }
+  return ws.isWord;
+};
+
+/**
+ * Returns if there is any word in the trie that starts with the given prefix.
+ * @param {string} prefix
+ * @return {boolean}
+ */
+Trie.prototype.startsWith = function(prefix) {
+  let ws = this.root;
+  for (let i = 0; i < prefix.length; i++) {
+    const c = prefix[i];
+    const current = computeIndex(c);
+    if (!ws.children[current]) return false;
+    ws = ws.children[current];
+  }
+  return true;
+};
+
+/**
+ * Your Trie object will be instantiated and called as such:
+ * var obj = new Trie()
+ * obj.insert(word)
+ * var param_2 = obj.search(word)
+ * var param_3 = obj.startsWith(prefix)
+ */
+```

problems/42.trapping-rain-water.md

+## 题目地址
+https://leetcode.com/problems/trapping-rain-water/description/
+
+## 题目描述
+
+
+```
+Given n non-negative integers representing an elevation map where the width of each bar is 1, compute how much water it is able to trap after raining.
+
+
+The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1]. In this case, 6 units of rain water (blue section) are being trapped. Thanks Marcos for contributing this image!
+
+
+```
+
+![42.trapping-rain-water-1](../assets/problems/42.trapping-rain-water-1.png)
+
+```
+Example:
+
+Input: [0,1,0,2,1,0,1,3,2,1,2,1]
+Output: 6
+
+```
+
+## 思路
+
+这是一道雨水收集的问题， 难度为`hard`. 如图所示，让我们求下过雨之后最多可以积攒多少的水。
+
+如果采用暴力求解的话，思路应该是height数组依次求和，然后相加。
+
+伪代码:
+
+```js
+
+for(let i = 0; i < height.length; i++) {
+    area += (h[i] - height[i]) * 1; // h为下雨之后的水位
+}
+
+```
+如上图那么h为 [1, 1, 2, 2, ,2 ,2, ,3, 2, 2, 2, 1]
+
+问题转化为求h，那么h[i]又等于`左右两侧柱子的最大值中的较小值`，即
+`h[i] = Math.min(左边柱子最大值, 右边柱子最大值)`
+
+问题的关键在于求解`左边柱子最大值`和`右边柱子最大值`,
+我们其实可以用两个数组来表示`leftMax`, `rightMax`，
+以leftMax为例，leftMax[i]代表i的左侧柱子的最大值，因此我们维护两个数组即可。
+## 关键点解析
+
+- 建模 `h[i] = Math.min(左边柱子最大值, 右边柱子最大值)`(h为下雨之后的水位)
+
+## 代码
+
+```js
+
+/*
+ * @lc app=leetcode id=42 lang=javascript
+ *
+ * [42] Trapping Rain Water
+ *
+ * https://leetcode.com/problems/trapping-rain-water/description/
+ *
+ * algorithms
+ * Hard (42.06%)
+ * Total Accepted:    278.1K
+ * Total Submissions: 651.6K
+ * Testcase Example:  '[0,1,0,2,1,0,1,3,2,1,2,1]'
+ *
+ * Given n non-negative integers representing an elevation map where the width
+ * of each bar is 1, compute how much water it is able to trap after raining.
+ * 
+ * 
+ * The above elevation map is represented by array [0,1,0,2,1,0,1,3,2,1,2,1].
+ * In this case, 6 units of rain water (blue section) are being trapped. Thanks
+ * Marcos for contributing this image!
+ * 
+ * Example:
+ * 
+ * 
+ * Input: [0,1,0,2,1,0,1,3,2,1,2,1]
+ * Output: 6
+ * 
+ */
+/**
+ * @param {number[]} height
+ * @return {number}
+ */
+var trap = function(height) {
+    let max = 0;
+    let volumn = 0;
+    const leftMax = [];
+    const rightMax = [];
+
+    for(let i = 0; i < height.length; i++) {
+        leftMax[i] = max = Math.max(height[i], max);
+    }
+
+    max = 0;
+
+    for(let i = height.length - 1; i >= 0; i--) {
+        rightMax[i] = max = Math.max(height[i], max);
+    }
+
+    for(let i = 0; i < height.length; i++) {
+        volumn = volumn +  Math.min(leftMax[i], rightMax[i]) - height[i]
+    }
+
+    return volumn;
+};
+
+```

problems/55.jump-game.md

+
+## 题目地址
+https://leetcode.com/problems/jump-game/description/
+
+## 题目描述
+```
+Given an array of non-negative integers, you are initially positioned at the first index of the array.
+
+Each element in the array represents your maximum jump length at that position.
+
+Determine if you are able to reach the last index.
+
+Example 1:
+
+Input: [2,3,1,1,4]
+Output: true
+Explanation: Jump 1 step from index 0 to 1, then 3 steps to the last index.
+Example 2:
+
+Input: [3,2,1,0,4]
+Output: false
+Explanation: You will always arrive at index 3 no matter what. Its maximum
+             jump length is 0, which makes it impossible to reach the last index.
+
+```
+
+## 思路
+
+这道题目是一道典型的`回溯`类型题目。
+思路就是用一个变量记录当前能够到达的最大的索引，我们逐个遍历数组中的元素去更新这个索引。
+变量完成判断这个索引是否大于数组下表即可。
+## 关键点解析
+
+- 建模 (记录和更新当前位置能够到达的最大的索引即可)
+
+## 代码
+
+```js
+
+/*
+ * @lc app=leetcode id=55 lang=javascript
+ *
+ * [55] Jump Game
+ *
+ * https://leetcode.com/problems/jump-game/description/
+ *
+ * algorithms
+ * Medium (31.38%)
+ * Total Accepted:    252.4K
+ * Total Submissions: 797.2K
+ * Testcase Example:  '[2,3,1,1,4]'
+ *
+ * Given an array of non-negative integers, you are initially positioned at the
+ * first index of the array.
+ *
+ * Each element in the array represents your maximum jump length at that
+ * position.
+ *
+ * Determine if you are able to reach the last index.
+ *
+ * Example 1:
+ *
+ *
+ * Input: [2,3,1,1,4]
+ * Output: true
+ * Explanation: Jump 1 step from index 0 to 1, then 3 steps to the last
+ * index.
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: [3,2,1,0,4]
+ * Output: false
+ * Explanation: You will always arrive at index 3 no matter what. Its
+ * maximum
+ * jump length is 0, which makes it impossible to reach the last index.
+ *
+ *
+ */
+/**
+ * @param {number[]} nums
+ * @return {boolean}
+ */
+var canJump = function(nums) {
+  let max = 0; // 能够走到的数组下标
+
+  for(let i = 0; i < nums.length; i++) {
+      if (max < i) return false; // 当前这一步都走不到，后面更走不到了
+      max = Math.max(nums[i] + i, max);
+  }
+
+  return max >= nums.length - 1
+};
+
+```

todo/10.regular-expression-matching.js

+/*
+ * @lc app=leetcode id=10 lang=javascript
+ *
+ * [10] Regular Expression Matching
+ *
+ * https://leetcode.com/problems/regular-expression-matching/description/
+ *
+ * algorithms
+ * Hard (25.00%)
+ * Total Accepted:    293.3K
+ * Total Submissions: 1.2M
+ * Testcase Example:  '"aa"\n"a"'
+ *
+ * Given an input string (s) and a pattern (p), implement regular expression
+ * matching with support for '.' and '*'.
+ * 
+ * 
+ * '.' Matches any single character.
+ * '*' Matches zero or more of the preceding element.
+ * 
+ * 
+ * The matching should cover the entire input string (not partial).
+ * 
+ * Note:
+ * 
+ * 
+ * s could be empty and contains only lowercase letters a-z.
+ * p could be empty and contains only lowercase letters a-z, and characters
+ * like . or *.
+ * 
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input:
+ * s = "aa"
+ * p = "a"
+ * Output: false
+ * Explanation: "a" does not match the entire string "aa".
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input:
+ * s = "aa"
+ * p = "a*"
+ * Output: true
+ * Explanation: '*' means zero or more of the precedeng element, 'a'.
+ * Therefore, by repeating 'a' once, it becomes "aa".
+ * 
+ * 
+ * Example 3:
+ * 
+ * 
+ * Input:
+ * s = "ab"
+ * p = ".*"
+ * Output: true
+ * Explanation: ".*" means "zero or more (*) of any character (.)".
+ * 
+ * 
+ * Example 4:
+ * 
+ * 
+ * Input:
+ * s = "aab"
+ * p = "c*a*b"
+ * Output: true
+ * Explanation: c can be repeated 0 times, a can be repeated 1 time. Therefore
+ * it matches "aab".
+ * 
+ * 
+ * Example 5:
+ * 
+ * 
+ * Input:
+ * s = "mississippi"
+ * p = "mis*is*p*."
+ * Output: false
+ * 
+ * 
+ */
+/**
+ * @param {string} s
+ * @param {string} p
+ * @return {boolean}
+ */
+var isMatch = function(s, p) {
+    
+};
+

todo/365.water-and-jug-problem.js

+/*
+ * @lc app=leetcode id=365 lang=javascript
+ *
+ * [365] Water and Jug Problem
+ *
+ * https://leetcode.com/problems/water-and-jug-problem/description/
+ *
+ * algorithms
+ * Medium (28.76%)
+ * Total Accepted:    27K
+ * Total Submissions: 93.7K
+ * Testcase Example:  '3\n5\n4'
+ *
+ * You are given two jugs with capacities x and y litres. There is an infinite
+ * amount of water supply available. You need to determine whether it is
+ * possible to measure exactly z litres using these two jugs.
+ * 
+ * If z liters of water is measurable, you must have z liters of water
+ * contained within one or both buckets by the end.
+ * 
+ * Operations allowed:
+ * 
+ * 
+ * Fill any of the jugs completely with water.
+ * Empty any of the jugs.
+ * Pour water from one jug into another till the other jug is completely full
+ * or the first jug itself is empty.
+ * 
+ * 
+ * Example 1: (From the famous "Die Hard" example)
+ * 
+ * 
+ * Input: x = 3, y = 5, z = 4
+ * Output: True
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: x = 2, y = 6, z = 5
+ * Output: False
+ * 
+ */
+/**
+ * @param {number} x
+ * @param {number} y
+ * @param {number} z
+ * @return {boolean}
+ */
+var canMeasureWater = function(x, y, z) {
+    const visited = {};
+};
+