20190119

code/lc494.java

+package code;
+/*
+ * 494. Target Sum
+ * 题意：给数组中的元素赋加减号，使得和为target的分配方案有几种
+ * 难度：Medium
+ * 分类：Dynamic Programming, Depth-first Search
+ * 思路：可以用递归+mem的方法。也可以转化为0，1背包问题，注意dp的时候把下标移位。另一种方法是转化为子数组的和为(target + sum(nums))/2的问题，求解方法类似lc416
+ * Tips：多抽象总结一下相关的问题，如何抽象出背包。对于这个数字，要么+，要么-，就两种情况。
+ */
+public class lc494 {
+    public int findTargetSumWays(int[] nums, int S) {
+        int sum = 0;
+        for (int i : nums) {
+            sum += i;
+        }
+        if(S>sum||S<-sum)
+            return 0;
+        int[] dp = new int[sum*2+1]; //还有0,所以+1
+        dp[sum] = 1;
+        for (int i = 0; i < nums.length ; i++) {
+            int[] dp2 = new int[sum*2+1];
+            for (int j = 0; j < dp.length ; j++) {
+                if(j+nums[i]<dp.length)     //判断下别越界了
+                    dp2[j+nums[i]] += dp[j];       // +=dp[j]  求得是总数
+                if(j-nums[i]>=0)
+                    dp2[j-nums[i]] += dp[j];
+            }
+            dp = dp2;
+        }
+        return dp[S+sum];
+    }
+}

code/lc621.java

+package code;
+/*
+ * 621. Task Scheduler
+ * 题意：一系列任务，相同任务之间至少有n个时间间隔，问最少执行时间
+ * 难度：Medium
+ * 分类：Array, Greedy, Tree
+ * 思路：统计出出现次数最多的那个任务的出现次数，再统计下和他相同次数的任务个数，返回 (max-1)*(n+1)+count 即可
+ * Tips：
+ */
+public class lc621 {
+    public int leastInterval(char[] tasks, int n) {
+        int[] map = new int[26];
+        int max = 0;
+        for (char ch : tasks ) {
+            map[ch-'A'] += 1;
+            max = Math.max(map[ch-'A'], max);
+        }
+        int count = 0;
+        for(int i: map){
+            if(i==max) count++;
+        }
+        return Math.max(tasks.length, (max-1)*(n+1)+count );    // max-1 不算最后一个周期， 最后一个周期时间是count，间隔为n，周期为n+1
+    }
+}

code/lc647.java

+package code;
+/*
+ * 647. Palindromic Substrings
+ * 题意：一个字符串里有多少个回文子串
+ * 难度：Medium
+ * 分类：String, Dynamic Programming
+ * 思路：时间为N^2，用二维dp空间复杂度是N^2. 该题直接让判断是否回文，直接选择找中心字符，向两边拓展，空间为O(1)
+ * Tips：
+ */
+public class lc647 {
+    public static void main(String[] args) {
+        System.out.println(countSubstrings("aaa"));
+    }
+    public static int countSubstrings(String s) {
+        int count = 0;
+        for (int i = 0; i < s.length() ; i++) {
+            for (int j = 0; j <2 ; j++) {   //中心可能是两个字符，也可能是一个字符
+                int left = i;
+                int right = left+j;
+                while( left>=0 && right<s.length() && s.charAt(left)==s.charAt(right)){
+                    count++;
+                    left--;
+                    right++;
+                }
+            }
+        }
+        return count;
+    }
+}

readme.md

@@ -106,13 +106,13 @@ Welcome to improve this project with me.*
 | 337  | [House Robber III](https://leetcode.com/problems/house-robber-iii/) |Medium| [Java](./code/lc337.java)
 | 338  | [Counting Bits](https://leetcode.com/problems/counting-bits/) |Medium| [Java](./code/lc338.java)
 | 347  | [Top K Frequent Elements](https://leetcode.com/problems/top-k-frequent-elements/) |Medium| [Java](./code/lc347.java)
-| 394  | [Decode String](https://leetcode.com/problems/decode-string/) |Medium| [Java]
-| 406  | [Queue Reconstruction by Height](https://leetcode.com/problems/queue-reconstruction-by-height/) |Medium| [Java]
-| 416  | [Partition Equal Subset Sum](https://leetcode.com/problems/partition-equal-subset-sum/) |Medium| [Java]
-| 437  | [Path Sum III](https://leetcode.com/problems/path-sum-iii/)  | Easy |[Java]
+| 394  | [Decode String](https://leetcode.com/problems/decode-string/) |Medium| [Java](./code/lc394.java)
+| 406  | [Queue Reconstruction by Height](https://leetcode.com/problems/queue-reconstruction-by-height/) |Medium| [Java](./code/lc406.java)
+| 416  | [Partition Equal Subset Sum](https://leetcode.com/problems/partition-equal-subset-sum/) |Medium| [Java](./code/lc416.java)
+| 437  | [Path Sum III](https://leetcode.com/problems/path-sum-iii/)  | Easy |[Java](./code/lc437.java)
 | 438  | [Find All Anagrams in a String](https://leetcode.com/problems/find-all-anagrams-in-a-string/) | Easy |[Java](./code/lc438.java)
 | 448  | [Find All Numbers Disappeared in an Array](https://leetcode.com/problems/find-all-numbers-disappeared-in-an-array/) | Easy |  [Java]
-| 461  | [Hamming Distance](https://leetcode.com/problems/hamming-distance/) | Easy |[Java]
+| 461  | [Hamming Distance](https://leetcode.com/problems/hamming-distance/) | Easy |[Java](./code/lc494.java)
 | 494  | [Target Sum](https://leetcode.com/problems/target-sum/) |Medium|[Java]
 | 538  | [Convert BST to Greater Tree](https://leetcode.com/problems/convert-bst-to-greater-tree/description/) | Easy |[Java]
 | 543  | [Diameter of Binary Tree](https://leetcode.com/problems/diameter-of-binary-tree/description/) | Easy | [Java]
@@ -120,6 +120,6 @@ Welcome to improve this project with me.*
 | 572  | [Subtree of Another Tree](https://leetcode.com/problems/subtree-of-another-tree/description/) | Easy | [Java]
 | 581  | [Shortest Unsorted Continuous Subarray](https://leetcode.com/problems/shortest-unsorted-continuous-subarray/description/) | Easy | [Java]
 | 617  | [Merge Two Binary Trees](https://leetcode.com/problems/merge-two-binary-trees/description/) | Easy | [Java]
-| 621  | [Task Scheduler](https://leetcode.com/problems/task-scheduler) |Medium|  [Java] |
-| 647  | [Palindromic Substrings](https://leetcode.com/problems/palindromic-substrings) |Medium|[Java]
+| 621  | [Task Scheduler](https://leetcode.com/problems/task-scheduler) |Medium|  [Java](./code/lc621.java)
+| 647  | [Palindromic Substrings](https://leetcode.com/problems/palindromic-substrings) |Medium|[Java](./code/lc647.java)