Merge branch 'master' into patch-4

Author: youngyangyang04

.DS_Store

@@ -118,6 +118,18 @@ class Solution:
                 return [records[target - val], idx] # 如果存在就返回字典记录索引和当前索引
 ```
 
+Python (v2):
+
+```python
+class Solution:
+    def twoSum(self, nums: List[int], target: int) -> List[int]:
+        rec = {}
+        for i in range(len(nums)):
+            rest = target - nums[i]
+            # Use get to get the index of the data, making use of one of the dictionary properties.
+            if rec.get(rest, None) is not None: return [rec[rest], i]
+            rec[nums[i]] = i
+```
 
 Go：
 

problems/0001.两数之和.md

@@ -260,7 +260,26 @@ public:
 
 # 其他语言版本
 
-## Java
+Java：
+
+```java
+public int[] twoSum(int[] nums, int target) {
+    int[] res = new int[2];
+    if(nums == null || nums.length == 0){
+        return res;
+    }
+    Map<Integer, Integer> map = new HashMap<>();
+    for(int i = 0; i < nums.length; i++){
+        int temp = target - nums[i];
+        if(map.containsKey(temp)){
+            res[1] = i;
+            res[0] = map.get(temp);
+        }
+        map.put(nums[i], i);
+    }
+    return res;
+}
+```
 
 ```java
 // 双指针 中心扩散法
@@ -291,7 +310,7 @@ class Solution {
 }
 ```
 
-## Python
+Python：
 
 ```python
 class Solution:
@@ -312,7 +331,8 @@ class Solution:
         return s[left:right + 1]
 
 ```
-> 双指针法：
+双指针：
+
 ```python
 class Solution:
     def longestPalindrome(self, s: str) -> str:
@@ -340,13 +360,13 @@ class Solution:
         return s[start:end]
 
 ```
-## Go
+Go：
 
 ```go
 
 ```
 
-## JavaScript
+JavaScript：
 
 ```js
 //动态规划解法
@@ -462,8 +482,9 @@ var longestPalindrome = function(s) {
 };
 ```
 
-## C
-动态规划:
+C：
+
+动态规划：
 ```c
 //初始化dp数组，全部初始为false
 bool **initDP(int strLen) {
@@ -513,7 +534,7 @@ char * longestPalindrome(char * s){
 }
 ```
 
-双指针:
+双指针：
 ```c
 int left, maxLength;
 void extend(char *str, int i, int j, int size) {

problems/0005.最长回文子串.md

@@ -247,7 +247,34 @@ class Solution:
                     right -= 1
         return ans
 ```
+Python (v2):
+
+```python
+class Solution:
+    def threeSum(self, nums: List[int]) -> List[List[int]]:
+        if len(nums) < 3: return []
+        nums, res = sorted(nums), []
+        for i in range(len(nums) - 2):
+            cur, l, r = nums[i], i + 1, len(nums) - 1
+            if res != [] and res[-1][0] == cur: continue # Drop duplicates for the first time.
+
+            while l < r:
+                if cur + nums[l] + nums[r] == 0:
+                    res.append([cur, nums[l], nums[r]])
+                    # Drop duplicates for the second time in interation of l & r. Only used when target situation occurs, because that is the reason for dropping duplicates.
+                    while l < r - 1 and nums[l] == nums[l + 1]:
+                        l += 1
+                    while r > l + 1 and nums[r] == nums[r - 1]:
+                        r -= 1
+                if cur + nums[l] + nums[r] > 0:
+                    r -= 1
+                else:
+                    l += 1
+        return res
+```
+
 Go：
+
 ```Go
 func threeSum(nums []int)[][]int{
 	sort.Ints(nums)

problems/0015.三数之和.md

@@ -216,6 +216,7 @@ class Solution(object):
 
         # good thing about using python is you can use set to drop duplicates.
         ans = set()
+        # ans = []  # save results by list()
         for i in range(len(nums)):
             for j in range(i + 1, len(nums)):
                 for k in range(j + 1, len(nums)):
@@ -224,10 +225,16 @@ class Solution(object):
                         # make sure no duplicates.
                         count = (nums[i] == val) + (nums[j] == val) + (nums[k] == val)
                         if hashmap[val] > count:
-                            ans.add(tuple(sorted([nums[i], nums[j], nums[k], val])))
-                    else:
-                        continue
-        return ans
+                          ans_tmp = tuple(sorted([nums[i], nums[j], nums[k], val]))
+                          ans.add(ans_tmp)
+                          # Avoiding duplication in list manner but it cause time complexity increases
+                          # if ans_tmp not in ans:  
+                          #     ans.append(ans_tmp)
+                        else:
+                            continue
+        return list(ans) 
+        # if used list() to save results, just 
+        # return ans
 
 ```
 

problems/0018.四数之和.md

@@ -254,32 +254,20 @@ TypeScript：
 
 ```typescript
 function swapPairs(head: ListNode | null): ListNode | null {
-    /**
-     * 初始状态：
-     * curNode -> node1 -> node2 -> tmepNode
-     * 转换过程：
-     * curNode -> node2
-     * curNode -> node2 -> node1
-     * curNode -> node2 -> node1 -> tempNode
-     * curNode = node1
-     */
-    let retNode: ListNode | null = new ListNode(0, head),
-        curNode: ListNode | null = retNode,
-        node1: ListNode | null = null,
-        node2: ListNode | null = null,
-        tempNode: ListNode | null = null;
-
-    while (curNode && curNode.next && curNode.next.next) {
-        node1 = curNode.next;
-        node2 = curNode.next.next;
-        tempNode = node2.next;
-        curNode.next = node2;
-        node2.next = node1;
-        node1.next = tempNode;
-        curNode = node1;
-    }
-    return retNode.next;
-};
+  const dummyHead: ListNode = new ListNode(0, head);
+  let cur: ListNode = dummyHead;
+  while(cur.next !== null && cur.next.next !== null) {
+    const tem: ListNode = cur.next;
+    const tem1: ListNode = cur.next.next.next;
+    
+    cur.next = cur.next.next; // step 1
+    cur.next.next = tem; // step 2
+    cur.next.next.next = tem1; // step 3
+    
+    cur = cur.next.next;
+  }
+  return dummyHead.next;
+}
 ```
 
 Kotlin:

problems/0024.两两交换链表中的节点.md

@@ -365,7 +365,7 @@ public:
 
 ## 其他语言版本 
 
-Java: 
+### Java: 
 
 双指针法
 ```java
@@ -468,7 +468,7 @@ class Solution {
 }
 ```
 
-Python:
+### Python:
 
 双指针法
 ```python3
@@ -575,7 +575,7 @@ class Solution:
 
 ```
 
-Go:
+### Go 
 
 ```go
 func trap(height []int) int {
@@ -642,7 +642,7 @@ func min(a,b int)int{
 
 
 
-JavaScript:
+### JavaScript:
 
 ```javascript
 //双指针
@@ -744,7 +744,7 @@ var trap = function(height) {
 };
 ```
 
-C:
+### C:
 
 一种更简便的双指针方法：
 

problems/0042.接雨水.md

@@ -174,6 +174,7 @@ const maxSubArray = nums => {
     // 数组长度，dp初始化
     const len = nums.length;
     let dp = new Array(len).fill(0);
+    dp[0] = nums[0];
     // 最大值初始化为dp[0]
     let max = dp[0];
     for (let i = 1; i < len; i++) {

problems/0053.最大子序和（动态规划）.md

@@ -192,47 +192,31 @@ python3:
 
 ```python
 class Solution:
-
     def generateMatrix(self, n: int) -> List[List[int]]:
-        # 初始化要填充的正方形
-        matrix = [[0] * n for _ in range(n)]
-
-        left, right, up, down = 0, n - 1, 0, n - 1
-        number = 1  # 要填充的数字
-
-        while left < right and up < down:
-
-            # 从左到右填充上边
-            for x in range(left, right):
-                matrix[up][x] = number
-                number += 1
-
-            # 从上到下填充右边
-            for y in range(up, down):
-                matrix[y][right] = number
-                number += 1
-
-            # 从右到左填充下边
-            for x in range(right, left, -1):
-                matrix[down][x] = number
-                number += 1
-
-            # 从下到上填充左边
-            for y in range(down, up, -1):
-                matrix[y][left] = number
-                number += 1
-
-            # 缩小要填充的范围
-            left += 1
-            right -= 1
-            up += 1
-            down -= 1
-
-        # 如果阶数为奇数，额外填充一次中心
-        if n % 2:
-            matrix[n // 2][n // 2] = number
-
-        return matrix
+        nums = [[0] * n for _ in range(n)]
+        startx, starty = 0, 0               # 起始点
+        loop, mid = n // 2, n // 2          # 迭代次数、n为奇数时，矩阵的中心点
+        count = 1                           # 计数
+
+        for offset in range(1, loop + 1) :      # 每循环一层偏移量加1，偏移量从1开始
+            for i in range(starty, n - offset) :    # 从左至右，左闭右开
+                nums[startx][i] = count
+                count += 1
+            for i in range(startx, n - offset) :    # 从上至下
+                nums[i][n - offset] = count
+                count += 1
+            for i in range(n - offset, starty, -1) : # 从右至左
+                nums[n - offset][i] = count
+                count += 1
+            for i in range(n - offset, startx, -1) : # 从下至上
+                nums[i][starty] = count
+                count += 1                
+            startx += 1         # 更新起始点
+            starty += 1
+
+        if n % 2 != 0 :			# n为奇数时，填充中心点
+            nums[mid][mid] = count 
+        return nums
 ```
 
 javaScript

problems/0059.螺旋矩阵II.md

@@ -155,6 +155,39 @@ public:
 * 时间复杂度：$O(n × m)$，n、m 分别为obstacleGrid 长度和宽度
 * 空间复杂度：$O(n × m)$
 
+
+同样我们给出空间优化版本：
+```CPP
+class Solution {
+public:
+    int uniquePathsWithObstacles(vector<vector<int>>& obstacleGrid) {
+        if (obstacleGrid[0][0] == 1)
+            return 0;
+        vector<int> dp(obstacleGrid[0].size());
+        for (int j = 0; j < dp.size(); ++j)
+            if (obstacleGrid[0][j] == 1)
+                dp[j] = 0;
+            else if (j == 0)
+                dp[j] = 1;
+            else
+                dp[j] = dp[j-1];
+
+        for (int i = 1; i < obstacleGrid.size(); ++i)
+            for (int j = 0; j < dp.size(); ++j){
+                if (obstacleGrid[i][j] == 1)
+                    dp[j] = 0;
+                else if (j != 0)
+                    dp[j] = dp[j] + dp[j-1];
+            }
+        return dp.back();
+    }
+};
+```
+
+* 时间复杂度：$O(n × m)$，n、m 分别为obstacleGrid 长度和宽度
+* 空间复杂度：$O(m)$
+
+
 ## 总结
 
 本题是[62.不同路径](https://programmercarl.com/0062.不同路径.html)的障碍版，整体思路大体一致。