feat: 部分文件增加内容

152.maximum-product-subarray.js

+/*
+ * @lc app=leetcode id=152 lang=javascript
+ *
+ * [152] Maximum Product Subarray
+ *
+ * https://leetcode.com/problems/maximum-product-subarray/description/
+ *
+ * algorithms
+ * Medium (28.61%)
+ * Total Accepted:    202.8K
+ * Total Submissions: 700K
+ * Testcase Example:  '[2,3,-2,4]'
+ *
+ * Given an integer array nums, find the contiguous subarray within an array
+ * (containing at least one number) which has the largest product.
+ *
+ * Example 1:
+ *
+ *
+ * Input: [2,3,-2,4]
+ * Output: 6
+ * Explanation: [2,3] has the largest product 6.
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: [-2,0,-1]
+ * Output: 0
+ * Explanation: The result cannot be 2, because [-2,-1] is not a subarray.
+ *
+ */
+/**
+ * @param {number[]} nums
+ * @return {number}
+ */
+var maxProduct = function(nums) {
+  // let max = nums[0];
+  // let temp = null;
+  // for(let i = 0; i < nums.length; i++) {
+  //     temp = nums[i];
+  //     max = Math.max(temp, max);
+  //     for(let j = i + 1; j < nums.length; j++) {
+  //         temp *= nums[j];
+  //         max = Math.max(temp, max);
+  //     }
+  // }
+
+  // return max;
+  let max = nums[0];
+  let min = nums[0];
+  let res = nums[0];
+
+  for (let i = 1; i < nums.length; i++) {
+    let tmp = min;
+    min = Math.min(nums[i], Math.min(max * nums[i], min * nums[i])); // 取最小
+    max = Math.max(nums[i], Math.max(max * nums[i], tmp * nums[i])); /// 取最大
+    res = Math.max(res, max);
+  }
+  return res;
+};

189.rotate-array.js

+/*
+ * @lc app=leetcode id=189 lang=javascript
+ *
+ * [189] Rotate Array
+ *
+ * https://leetcode.com/problems/rotate-array/description/
+ *
+ * algorithms
+ * Easy (29.07%)
+ * Total Accepted:    287.3K
+ * Total Submissions: 966.9K
+ * Testcase Example:  '[1,2,3,4,5,6,7]\n3'
+ *
+ * Given an array, rotate the array to the right by k steps, where k is
+ * non-negative.
+ *
+ * Example 1:
+ *
+ *
+ * Input: [1,2,3,4,5,6,7] and k = 3
+ * Output: [5,6,7,1,2,3,4]
+ * Explanation:
+ * rotate 1 steps to the right: [7,1,2,3,4,5,6]
+ * rotate 2 steps to the right: [6,7,1,2,3,4,5]
+ * rotate 3 steps to the right: [5,6,7,1,2,3,4]
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: [-1,-100,3,99] and k = 2
+ * Output: [3,99,-1,-100]
+ * Explanation:
+ * rotate 1 steps to the right: [99,-1,-100,3]
+ * rotate 2 steps to the right: [3,99,-1,-100]
+ *
+ *
+ * Note:
+ *
+ *
+ * Try to come up as many solutions as you can, there are at least 3 different
+ * ways to solve this problem.
+ * Could you do it in-place with O(1) extra space?
+ *
+ */
+/**
+ * @param {number[]} nums
+ * @param {number} k
+ * @return {void} Do not return anything, modify nums in-place instead.
+ */
+var rotate = function(nums, k) {
+  // 就像扩容一样操作
+  k = k % nums.length;
+  const n = nums.length;
+
+  for (let i = nums.length - 1; i >= 0; i--) {
+    nums[i + k] = nums[i];
+  }
+
+  for (let i = 0; i < k; i++) {
+    nums[i] = nums[n + i];
+  }
+  nums.length = n;
+};

217.contains-duplicate.js

+/*
+ * @lc app=leetcode id=217 lang=javascript
+ *
+ * [217] Contains Duplicate
+ *
+ * https://leetcode.com/problems/contains-duplicate/description/
+ *
+ * algorithms
+ * Easy (50.92%)
+ * Total Accepted:    324K
+ * Total Submissions: 628.5K
+ * Testcase Example:  '[1,2,3,1]'
+ *
+ * Given an array of integers, find if the array contains any duplicates.
+ * 
+ * Your function should return true if any value appears at least twice in the
+ * array, and it should return false if every element is distinct.
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: [1,2,3,1]
+ * Output: true
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: [1,2,3,4]
+ * Output: false
+ * 
+ * Example 3:
+ * 
+ * 
+ * Input: [1,1,1,3,3,4,3,2,4,2]
+ * Output: true
+ * 
+ */
+/**
+ * @param {number[]} nums
+ * @return {boolean}
+ */
+var containsDuplicate = function(nums) {
+    // 1. 暴力两层循环两两比较， 时间复杂度O(n^2) 空间复杂度O(1)
+
+    // 2. 先排序，之后比较前后元素是否一致即可，一层循环即可，如果排序使用的比较排序的话时间复杂度O(nlogn) 空间复杂度O(1)
+
+    // 3. 用hashmap ，时间复杂度O(n) 空间复杂度O(n)
+    const visited = {};
+    for(let i = 0; i < nums.length; i++) {
+        if (visited[nums[i]]) return true;
+        visited[nums[i]] = true;
+    }
+    return false;
+};
+

23.merge-k-sorted-lists.js

+/*
+ * @lc app=leetcode id=23 lang=javascript
+ *
+ * [23] Merge k Sorted Lists
+ *
+ * https://leetcode.com/problems/merge-k-sorted-lists/description/
+ *
+ * algorithms
+ * Hard (33.14%)
+ * Total Accepted:    373.7K
+ * Total Submissions: 1.1M
+ * Testcase Example:  '[[1,4,5],[1,3,4],[2,6]]'
+ *
+ * Merge k sorted linked lists and return it as one sorted list. Analyze and
+ * describe its complexity.
+ *
+ * Example:
+ *
+ *
+ * Input:
+ * [
+ * 1->4->5,
+ * 1->3->4,
+ * 2->6
+ * ]
+ * Output: 1->1->2->3->4->4->5->6
+ *
+ *
+ */
+function mergeTwoLists(l1, l2) {
+  const dummyHead = {};
+  let current = dummyHead;
+  // 1 -> 3 -> 5
+  // 2 -> 4 -> 6
+  while (l1 !== null && l2 !== null) {
+    if (l1.val < l2.val) {
+      current.next = l1; // 把小的添加到结果链表
+      current = current.next; // 移动结果链表的指针
+      l1 = l1.next; // 移动小的那个链表的指针
+    } else {
+      current.next = l2;
+      current = current.next;
+      l2 = l2.next;
+    }
+  }
+
+  if (l1 === null) {
+    current.next = l2;
+  } else {
+    current.next = l1;
+  }
+  return dummyHead.next;
+}
+/**
+ * Definition for singly-linked list.
+ * function ListNode(val) {
+ *     this.val = val;
+ *     this.next = null;
+ * }
+ */
+/**
+ * @param {ListNode[]} lists
+ * @return {ListNode}
+ */
+var mergeKLists = function(lists) {
+  // 图参考： https://zhuanlan.zhihu.com/p/61796021
+  if (lists.length === 0) return null;
+  if (lists.length === 1) return lists[0];
+  if (lists.length === 2) {
+    return mergeTwoLists(lists[0], lists[1]);
+  }
+
+  const mid = lists.length >> 1;
+  const l1 = [];
+  for (let i = 0; i < mid; i++) {
+    l1[i] = lists[i];
+  }
+
+  const l2 = [];
+  for (let i = mid, j = 0; i < lists.length; i++, j++) {
+    l2[j] = lists[i];
+  }
+
+  return mergeTwoLists(mergeKLists(l1), mergeKLists(l2));
+};

236.lowest-common-ancestor-of-a-binary-tree.js

+/*
+ * @lc app=leetcode id=236 lang=javascript
+ *
+ * [236] Lowest Common Ancestor of a Binary Tree
+ *
+ * https://leetcode.com/problems/lowest-common-ancestor-of-a-binary-tree/description/
+ *
+ * algorithms
+ * Medium (35.63%)
+ * Total Accepted:    267.3K
+ * Total Submissions: 729.2K
+ * Testcase Example:  '[3,5,1,6,2,0,8,null,null,7,4]\n5\n1'
+ *
+ * Given a binary tree, find the lowest common ancestor (LCA) of two given
+ * nodes in the tree.
+ *
+ * According to the definition of LCA on Wikipedia: “The lowest common ancestor
+ * is defined between two nodes p and q as the lowest node in T that has both p
+ * and q as descendants (where we allow a node to be a descendant of itself).”
+ *
+ * Given the following binary tree:  root = [3,5,1,6,2,0,8,null,null,7,4]
+ *
+ *
+ *
+ * Example 1:
+ *
+ *
+ * Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 1
+ * Output: 3
+ * Explanation: The LCA of nodes 5 and 1 is 3.
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: root = [3,5,1,6,2,0,8,null,null,7,4], p = 5, q = 4
+ * Output: 5
+ * Explanation: The LCA of nodes 5 and 4 is 5, since a node can be a descendant
+ * of itself according to the LCA definition.
+ *
+ *
+ *
+ *
+ * Note:
+ *
+ *
+ * All of the nodes' values will be unique.
+ * p and q are different and both values will exist in the binary tree.
+ *
+ *
+ */
+/**
+ * Definition for a binary tree node.
+ * function TreeNode(val) {
+ *     this.val = val;
+ *     this.left = this.right = null;
+ * }
+ */
+/**
+ * @param {TreeNode} root
+ * @param {TreeNode} p
+ * @param {TreeNode} q
+ * @return {TreeNode}
+ */
+var lowestCommonAncestor = function(root, p, q) {
+  if (!root || root === p || root === q) return root;
+  const left = lowestCommonAncestor(root.left, p, q);
+  const right = lowestCommonAncestor(root.right, p, q);
+  if (!left) return right; // 左子树找不到，返回右子树
+  if (!right) return left; // 右子树找不到，返回左子树
+  return root; // 左右子树分别有一个，则返回root
+};

238.product-of-array-except-self.js

+/*
+ * @lc app=leetcode id=238 lang=javascript
+ *
+ * [238] Product of Array Except Self
+ *
+ * https://leetcode.com/problems/product-of-array-except-self/description/
+ *
+ * algorithms
+ * Medium (53.97%)
+ * Total Accepted:    246.5K
+ * Total Submissions: 451.4K
+ * Testcase Example:  '[1,2,3,4]'
+ *
+ * Given an array nums of n integers where n > 1,  return an array output such
+ * that output[i] is equal to the product of all the elements of nums except
+ * nums[i].
+ *
+ * Example:
+ *
+ *
+ * Input:  [1,2,3,4]
+ * Output: [24,12,8,6]
+ *
+ *
+ * Note: Please solve it without division and in O(n).
+ *
+ * Follow up:
+ * Could you solve it with constant space complexity? (The output array does
+ * not count as extra space for the purpose of space complexity analysis.)
+ *
+ */
+/**
+ * @param {number[]} nums
+ * @return {number[]}
+ */
+var productExceptSelf = function(nums) {
+  const ret = [];
+
+  for (let i = 0, temp = 1; i < nums.length; i++) {
+    ret[i] = temp;
+    temp *= nums[i];
+  }
+  // 此时ret[i]存放的是前i个元素相乘的结果(不包含第i个)
+
+  // 如果没有上面的循环的话，
+  // ret经过下面的循环会变成ret[i]存放的是后i个元素相乘的结果(不包含第i个)
+
+  // 我们的目标是ret[i]存放的所有数字相乘的结果(不包含第i个)
+
+  // 因此我们只需要对于上述的循环产生的ret[i]基础上运算即可
+  for (let i = nums.length - 1, temp = 1; i >= 0; i--) {
+    ret[i] *= temp;
+    temp *= nums[i];
+  }
+  return ret;
+};

326.power-of-three.js

+/*
+ * @lc app=leetcode id=326 lang=javascript
+ *
+ * [326] Power of Three
+ *
+ * https://leetcode.com/problems/power-of-three/description/
+ *
+ * algorithms
+ * Easy (41.43%)
+ * Total Accepted:    178.8K
+ * Total Submissions: 430.4K
+ * Testcase Example:  '27'
+ *
+ * Given an integer, write a function to determine if it is a power of three.
+ *
+ * Example 1:
+ *
+ *
+ * Input: 27
+ * Output: true
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: 0
+ * Output: false
+ *
+ * Example 3:
+ *
+ *
+ * Input: 9
+ * Output: true
+ *
+ * Example 4:
+ *
+ *
+ * Input: 45
+ * Output: false
+ *
+ * Follow up:
+ * Could you do it without using any loop / recursion?
+ */
+/**
+ * @param {number} n
+ * @return {boolean}
+ */
+var isPowerOfThree = function(n) {
+  // let i = 0;
+  // while(Math.pow(3, i) < n) {
+  //     i++;
+  // }
+  // return Math.pow(3, i) === n;
+
+  // 巧用整除
+  return n > 0 && Math.pow(3, 19) % n === 0;
+};

334.increasing-triplet-subsequence.js

+/*
+ * @lc app=leetcode id=334 lang=javascript
+ *
+ * [334] Increasing Triplet Subsequence
+ *
+ * https://leetcode.com/problems/increasing-triplet-subsequence/description/
+ *
+ * algorithms
+ * Medium (39.47%)
+ * Total Accepted:    89.6K
+ * Total Submissions: 226.6K
+ * Testcase Example:  '[1,2,3,4,5]'
+ *
+ * Given an unsorted array return whether an increasing subsequence of length 3
+ * exists or not in the array.
+ * 
+ * Formally the function should:
+ * 
+ * Return true if there exists i, j, k 
+ * such that arr[i] < arr[j] < arr[k] given 0 ≤ i < j < k ≤ n-1 else return
+ * false.
+ * 
+ * Note: Your algorithm should run in O(n) time complexity and O(1) space
+ * complexity.
+ * 
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: [1,2,3,4,5]
+ * Output: true
+ * 
+ * 
+ *
+ * Example 2:
+ * 
+ * 
+ * Input: [5,4,3,2,1]
+ * Output: false
+ * 
+ * 
+ * 
+ */
+/**
+ * @param {number[]} nums
+ * @return {boolean}
+ */
+var increasingTriplet = function(nums) {
+    if (nums.length < 3) return false;
+    let n1 = Number.MAX_VALUE;
+    let n2 = Number.MAX_VALUE;
+
+    for(let i = 0; i < nums.length; i++) {
+        if (nums[i] <= n1) {
+            n1 = nums[i]
+        } else if (nums[i] <= n2) {
+            n2 = nums[i]
+        } else {
+            return true;
+        }
+    }
+
+    return false;
+};
+

344.reverse-string.js

+/*
+ * @lc app=leetcode id=344 lang=javascript
+ *
+ * [344] Reverse String
+ *
+ * https://leetcode.com/problems/reverse-string/description/
+ *
+ * algorithms
+ * Easy (62.81%)
+ * Total Accepted:    409.9K
+ * Total Submissions: 649.5K
+ * Testcase Example:  '["h","e","l","l","o"]'
+ *
+ * Write a function that reverses a string. The input string is given as an
+ * array of characters char[].
+ * 
+ * Do not allocate extra space for another array, you must do this by modifying
+ * the input array in-place with O(1) extra memory.
+ * 
+ * You may assume all the characters consist of printable ascii
+ * characters.
+ * 
+ * 
+ * 
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: ["h","e","l","l","o"]
+ * Output: ["o","l","l","e","h"]
+ * 
+ * 
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: ["H","a","n","n","a","h"]
+ * Output: ["h","a","n","n","a","H"]
+ * 
+ * 
+ * 
+ * 
+ */
+/**
+ * @param {character[]} s
+ * @return {void} Do not return anything, modify s in-place instead.
+ */
+var reverseString = function(s) {
+    for(let i = 0; i < s.length >> 1; i++) {
+        const temp = s[i];
+        s[i] = s[s.length - i - 1];
+        s[s.length - i - 1] = temp;
+    }
+};
+

350.intersection-of-two-arrays-ii.js

+/*
+ * @lc app=leetcode id=350 lang=javascript
+ *
+ * [350] Intersection of Two Arrays II
+ *
+ * https://leetcode.com/problems/intersection-of-two-arrays-ii/description/
+ *
+ * algorithms
+ * Easy (46.84%)
+ * Total Accepted:    185.1K
+ * Total Submissions: 393.7K
+ * Testcase Example:  '[1,2,2,1]\n[2,2]'
+ *
+ * Given two arrays, write a function to compute their intersection.
+ *
+ * Example 1:
+ *
+ *
+ * Input: nums1 = [1,2,2,1], nums2 = [2,2]
+ * Output: [2,2]
+ *
+ *
+ *
+ * Example 2:
+ *
+ *
+ * Input: nums1 = [4,9,5], nums2 = [9,4,9,8,4]
+ * Output: [4,9]
+ *
+ *
+ * Note:
+ *
+ *
+ * Each element in the result should appear as many times as it shows in both
+ * arrays.
+ * The result can be in any order.
+ *
+ *
+ * Follow up:
+ *
+ *
+ * What if the given array is already sorted? How would you optimize your
+ * algorithm?
+ * What if nums1's size is small compared to nums2's size? Which algorithm is
+ * better?
+ * What if elements of nums2 are stored on disk, and the memory is limited such
+ * that you cannot load all elements into the memory at once?
+ *
+ *
+ */
+/**
+ * @param {number[]} nums1
+ * @param {number[]} nums2
+ * @return {number[]}
+ */
+var intersect = function(nums1, nums2) {
+  const res = [];
+
+  for (let i = 0; i < nums1.length; i++) {
+    if (nums2.includes(nums1[i])) { // 这里我们对两个数组排序，然后二分查找， 时间复杂度nlogn
+      nums2[nums2.indexOf(nums1[i])] = null;
+      res.push(nums1[i]);
+    }
+  }
+
+  return res;
+};

387.first-unique-character-in-a-string.js

+/*
+ * @lc app=leetcode id=387 lang=javascript
+ *
+ * [387] First Unique Character in a String
+ *
+ * https://leetcode.com/problems/first-unique-character-in-a-string/description/
+ *
+ * algorithms
+ * Easy (49.29%)
+ * Total Accepted:    255.6K
+ * Total Submissions: 513.8K
+ * Testcase Example:  '"leetcode"'
+ *
+ *
+ * Given a string, find the first non-repeating character in it and return it's
+ * index. If it doesn't exist, return -1.
+ *
+ * Examples:
+ *
+ * s = "leetcode"
+ * return 0.
+ *
+ * s = "loveleetcode",
+ * return 2.
+ *
+ *
+ *
+ *
+ * Note: You may assume the string contain only lowercase letters.
+ *
+ */
+/**
+ * @param {string} s
+ * @return {number}
+ */
+var firstUniqChar = function(s) {
+  for (let i = 0; i < s.length; i++) {
+    if (s.indexOf(s[i]) === s.lastIndexOf(s[i])) {
+      return i;
+    }
+  }
+  return -1;
+};

README.md

@@ -75,7 +75,7 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 
 #### 简单难度
 
-- [20. Valid Parentheses](./problems/20.validParentheses.md)
+- 🖊 [[20. Valid Parentheses](./problems/20.validParentheses.md)
 - [26.remove-duplicates-from-sorted-array](./problems/26.remove-duplicates-from-sorted-array.md)
 - 🆕 [88.merge-sorted-array](./problems/88.merge-sorted-array.md)
 - [136.single-number](./problems/136.single-number.md)
@@ -114,7 +114,7 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 - [103.binary-tree-zigzag-level-order-traversal](./problems/103.binary-tree-zigzag-level-order-traversal.md)
 - [139.word-break](./problems/139.word-breakmd)
 - [144.binary-tree-preorder-traversal](./problems/144.binary-tree-preorder-traversal.md)
-- [150.evaluate-reverse-polish-notation](./problems/150.evaluate-reverse-polish-notation.md)
+- 🖊 [[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)
@@ -142,9 +142,9 @@ leetcode 题解，记录自己的 leetcode 解题之路。
 
 ### 数据结构与算法的总结
 
-- 🖊 [basic-data-structure](./thinkings/basic-data-structure.md)（草稿）
-- [binary-tree-traversal](./thinkings/binary-tree-traversal.md)
-- [dynamic-programming](./thinkings/dynamic-programming.md)
+- 🖊 [数据结构](./thinkings/basic-data-structure.md)（草稿）
+- 🖊 [[二叉树的遍历](./thinkings/binary-tree-traversal.md)
+- [动态规划](./thinkings/dynamic-programming.md)
 - [哈夫曼编码和游程编码](./thinkings/run-length-encode-and-huffman-encode.md)
 - [布隆过滤器](./thinkings/bloom-filter.md)
 

problems/150.evaluate-reverse-polish-notation.md

@@ -100,21 +100,28 @@ The given RPN expression is always valid. That means the expression would always
  * @return {number}
  */
 var evalRPN = function(tokens) {
+  // 这种算法的前提是 tokens是有效的，
+  // 当然这由算法来保证
   const stack = [];
 
   for (let index = 0; index < tokens.length; index++) {
     const token = tokens[index];
+    // 对于运算数， 我们直接入栈
     if (!Number.isNaN(Number(token))) {
       stack.push(token);
     } else {
+      // 遇到操作符，我们直接大胆运算，不用考虑算术优先级
+      // 然后将运算结果入栈即可
+
+      // 当然如果题目进一步扩展，允许使用单目等其他运算符，我们的算法需要做微小的调整
       const a = Number(stack.pop());
       const b = Number(stack.pop());
       if (token === "*") {
-        stack.push(a * b);
+        stack.push(b * a);
       } else if (token === "/") {
-        stack.push(b / a > 0 ? Math.floor(b / a) : Math.ceil(b / a));
+        stack.push(b / a >> 0);
       } else if (token === "+") {
-        stack.push(a + b);
+        stack.push(b + a);
       } else if (token === "-") {
         stack.push(b - a);
       }
@@ -126,5 +133,10 @@ var evalRPN = function(tokens) {
 
 ```
 
+## 扩展
+
+逆波兰表达式中只改变运算符的顺序，并不会改变操作数的相对顺序，这是一个重要的性质。
+另外逆波兰表达式完全不关心操作符的优先级，这在中缀表达式中是做不到的，这很有趣，感兴趣的可以私下查找资料研究下为什么会这样。
+
 
 

problems/20.validParentheses.md

@@ -36,6 +36,8 @@ Output: true
 
 ## 思路
 
+关于这道题的思路，邓俊辉讲的非常好，没有看过的同学可以看一下, [视频地址](http://www.xuetangx.com/courses/course-v1:TsinghuaX+30240184+sp/courseware/ad1a23c053df4501a3facd66ef6ccfa9/8d6f450e7f7a445098ae1d507fda80f6/)。
+
 使用栈,遍历输入字符串
 
 如果当前字符为左半边括号时，则将其压入栈中
@@ -54,6 +56,11 @@ Output: true
 
 (图片来自： https://github.com/MisterBooo/LeetCodeAnimation)
 
+> 值得注意的是，如果题目要求只有一种括号，那么我们其实可以使用更简洁，更省内存的方式 - 计数器来进行求解，而
+不必要使用栈。
+
+> 事实上，这类问题还可以进一步扩展，我们可以去解析类似HTML等标记语法， 比如 <p></p> <body></body>
+
 ## 关键点解析
 
 1. 栈的基本特点和操作
@@ -155,4 +162,7 @@ var isValid = function(s) {
 
     return valid;
 };
-```
\ No newline at end of file
+```
+
+## 扩展
+如果让你检查XML标签是否闭合如何检查， 更进一步如果要你实现一个简单的XML的解析器，应该怎么实现？
\ No newline at end of file

problems/279.perfect-squares.md

@@ -78,7 +78,7 @@ DP 的代码见代码区。
 ```js
 for (let i = 1; i <= n; i++) {
   for (let j = 1; j * j <= i; j++) {
-    // 选（dp[i]） 还是  不选（dp[i - j * j]）
+    // 不选（dp[i]） 还是  选（dp[i - j * j]）
     dp[i] = Math.min(dp[i], dp[i - j * j] + 1);
   }
 }
@@ -130,7 +130,7 @@ var numSquares = function(n) {
   dp[0] = 0;
   for (let i = 1; i <= n; i++) {
     for (let j = 1; j * j <= i; j++) {
-      // 选（dp[i]） 还是  不选（dp[i - j * j]）
+      // 不选（dp[i]） 还是  选（dp[i - j * j]）
       dp[i] = Math.min(dp[i], dp[i - j * j] + 1);
     }
   }

thinkings/basic-data-structure.md

@@ -9,6 +9,8 @@
 数据结构我们可以从逻辑上分为线性结构和非线性结果。线性结构有
 数组，栈，链表等， 非线性结构有树，图等。
 
+> 其实我们可以称树为一种半线性结构。
+
 需要注意的是，线性和非线性不代表存储结构是线性的还是非线性的，这两者没有任何关系，它只是一种逻辑上的划分。
 比如我们可以用数组去存储二叉树。
 ### 数组
@@ -16,6 +18,8 @@
 数组是最简单的数据结构了，很多地方都用到它。 比如有一个数据列表等，用它是再合适不过了。
 其实后面的数据结构很多都有数组的影子。
 
+我们之后要讲的栈和队列其实都可以看成是一种`受限`的数组, 怎么个受限法呢？我们后面讨论。
+
 我们来讲几个有趣的例子来加深大家对数组这种数据结构的理解。
 #### React Hooks
 
@@ -69,6 +73,8 @@ React 将`如何确保组件内部hooks保存的状态之间的对应关系`这
 开发人员去保证，即你必须保证HOOKS的顺序严格一致，具体可以看React 官网关于 Hooks Rule 部分。
 ### 队列
 
+队列是一种受限的序列，它只能够操作队尾和队首，并且只能只能在队尾添加元素，在队首删除元素。
+
 队列作为一种最常见的数据结构同样有着非常广泛的应用， 比如消息队列
 
 > "队列"这个名称,可类比为现实生活中排队（不插队的那种）
@@ -107,6 +113,8 @@ React 将`如何确保组件内部hooks保存的状态之间的对应关系`这
 ![basic-data-structure-queue-2](../assets/thinkings/basic-data-structure-queue-2.png)
 
 ### 栈
+栈也是一种受限的序列，它只能够操作栈顶，不管入栈还是出栈，都是在栈顶操作。
+
 在计算机科学中, 一个 栈(stack) 是一种抽象数据类型,用作表示元素的集合,具有两种主要操作:
 
 push, 添加元素到栈的顶端(末尾);
@@ -155,6 +163,11 @@ foo();
 
 > 社区中有很多“执行上下文中的scope指的是执行栈中父级声明的变量”说法，这是完全错误的， JS是词法作用域，scope指的是函数定义时候的父级，和执行没关系
 
+
+栈常见的应用有进制转换，括号匹配，栈混洗，中缀表达式（用的很少），后缀表达式（逆波兰表达式）等。
+
+> 合法的栈混洗操作，其实和合法的括号匹配表达式之间存在着一一对应的关系，
+也就是说n个元素的栈混洗有多少种，n对括号的合法表达式就有多少种。感兴趣的可以查找相关资料
 ### 链表
 
 链表是一种最基本数据结构，熟练掌握链表的结构和常见操作是基础中的基础。
@@ -224,14 +237,66 @@ return, children, sibling也都是一个fiber，因此fiber看起来就是一个
 我目前也在写关于《从零开发react系列教程》中关于fiber架构的部分，如果你对具体实现感兴趣，欢迎关注。
 ### 非线性结构
 
+那么有了线性结果，我们为什么还需要非线性结果呢？ 答案是为了高效地兼顾静态操作和动态操作。
+大家可以对照各种数据结构的各种操作的复杂度来直观感受一下。
 ## 树
+树的应用同样非常广泛，小到文件系统，大到因特网，组织架构等都可以表示为树结构，
+而在我们前端眼中比较熟悉的DOM树也是一种树结构，而HTML作为一种DSL去描述这种树结构的具体表现形式。
+
+树其实是一种特殊的`图`，是一种无环连通图，是一种极大无环图，也是一种极小连通图。
+
+从另一个角度看，树是一种递归的数据结构。而且树的不同表示方法，比如不常用的`长子 + 兄弟`法，对于
+你理解树这种数据结构有着很大用处， 说是一种对树的本质的更深刻的理解也不为过。
+
+树的基本算法有前中后序遍历和层次遍历，有的同学对前中后这三个分别具体表现的访问顺序比较模糊，
+其实当初我也是一样的，后面我学到了一点，你只需要记住：`所谓的前中后指的是根节点的位置，其他位置按照先左后右排列即可`。
+比如前序遍历就是`根左右`, 中序就是`左根右`，后序就是`左右根`， 很简单吧？
+
+我刚才提到了树是一种递归的数据结构，因此树的遍历算法使用递归去完成非常简单，
+幸运的是树的算法基本上都要依赖于树的遍历。 但是递归在计算机中的性能一直都有问题，
+因此掌握不那么容易理解的"命令式地迭代"遍历算法在某些情况下是有用的。
+
+如果你使用迭代式方式去遍历的话，可以借助上面提到的`栈`来进行，可以极大减少代码量。
+
+> 如果使用栈来简化运算，由于栈是FILO的，因此一定要注意左右子树的推入顺序。
+
+树的重要性质：
+
+- 如果树有n个顶点，那么其就有n - 1条边，这说明了树的顶点数和边数是同阶的。
+- 任何一个节点到根节点存在`唯一`路径, 路径的长度为节点所处的深度
+
+
 
 ### 二叉树
 
+二叉树是节点度数不超过二的树，是树的一种特殊子集，有趣的是二叉树这种被限制的树结构却能够表示和实现所有的树，
+它背后的原理正是`长子 + 兄弟`法，用邓老师的话说就是`二叉树是多叉树的特例，但在有根且有序时，其描述能力却足以覆盖后者`。
+
+> 实际上， 在你使用`长子 + 兄弟`法表示树的同时，进行45度角旋转即可。 
+
+相关算法:
+
+- [94.binary-tree-inorder-traversal](../problems/94.binary-tree-inorder-traversal.md)
+- [102.binary-tree-level-order-traversal](../problems/102.binary-tree-level-order-traversal.md)
+- [103.binary-tree-zigzag-level-order-traversal](../problems/103.binary-tree-zigzag-level-order-traversal.md)
+- [144.binary-tree-preorder-traversal](../problems/144.binary-tree-preorder-traversal.md)
+- [145.binary-tree-postorder-traversal](../problems/145.binary-tree-postorder-traversal.md)
+- [199.binary-tree-right-side-view](../problems/199.binary-tree-right-side-view.md)
+
+相关概念：
+
+- 真二叉树
+
+
+另外我也专门开设了[二叉树的遍历](./binary-tree-traversal.md)章节, 具体细节和算法可以去那里查看。
 #### 堆
 
-优先级队列
+堆其实是一种优先级队列，在很多语言都有对应的内置数据结构，很遗憾javascript没有这种原生的数据结构。
+不过这对我们理解和运用不会有影响。
+
+相关算法：
 
+- [295.find-median-from-data-stream](../problems/295.find-median-from-data-stream.md)
 #### 二叉查找树
 
 ### 平衡树
@@ -244,4 +309,7 @@ database engine
 
 ### 字典树(前缀树)
 
+相关算法：
+
+- [208.implement-trie-prefix-tree](../problems/208.implement-trie-prefix-tree.md)
 ## 图

thinkings/binary-tree-traversal.md

@@ -38,6 +38,11 @@ BFS 的关键点在于如何记录每一层次是否遍历完成， 我们可以
 
 总结： 典型的递归数据结构，典型的用栈来简化操作的算法。
 
+其实从宏观上表现为：`自顶向下依次访问左侧链，然后自底向上依次访问右侧链`，
+如果从这个角度出发去写的话，算法就不一样了。从上向下我们可以直接递归访问即可，从下向上我们只需要借助栈也可以轻易做到。
+这种思路解题有点像我总结过的一个解题思路`backtrack` - 回溯法。这种思路有一个好处就是
+可以`统一三种遍历的思路`.
+
 ## 中序遍历
 
 相关问题[94.binary-tree-inorder-traversal](../problems/94.binary-tree-inorder-traversal.md)

todo/candidates/324.wiggle-sort-ii.js

+/*
+ * @lc app=leetcode id=324 lang=javascript
+ *
+ * [324] Wiggle Sort II
+ *
+ * https://leetcode.com/problems/wiggle-sort-ii/description/
+ *
+ * algorithms
+ * Medium (27.53%)
+ * Total Accepted:    57.5K
+ * Total Submissions: 206.9K
+ * Testcase Example:  '[1,5,1,1,6,4]'
+ *
+ * Given an unsorted array nums, reorder it such that nums[0] < nums[1] >
+ * nums[2] < nums[3]....
+ * 
+ * Example 1:
+ * 
+ * 
+ * Input: nums = [1, 5, 1, 1, 6, 4]
+ * Output: One possible answer is [1, 4, 1, 5, 1, 6].
+ * 
+ * Example 2:
+ * 
+ * 
+ * Input: nums = [1, 3, 2, 2, 3, 1]
+ * Output: One possible answer is [2, 3, 1, 3, 1, 2].
+ * 
+ * Note:
+ * You may assume all input has valid answer.
+ * 
+ * Follow Up:
+ * Can you do it in O(n) time and/or in-place with O(1) extra space?
+ */
+/**
+ * @param {number[]} nums
+ * @return {void} Do not return anything, modify nums in-place instead.
+ */
+var wiggleSort = function(nums) {
+    for(let i = 0; i < nums.length; i++) {}
+};
+