Merge branch 'master' of https://github.com/morningsky/leetcode-master

Author: morningsky

problems/0001.两数之和.md

@@ -136,6 +136,21 @@ func twoSum(nums []int, target int) []int {
 }
 ```
 
+```go
+// 使用map方式解题，降低时间复杂度
+func twoSum(nums []int, target int) []int {
+    m := make(map[int]int)
+    for index, val := range nums {
+        if preIndex, ok := m[target-val]; ok {
+            return []int{preIndex, index}
+        } else {
+            m[val] = index
+        }
+    }
+    return []int{}
+}
+```
+
 Rust
 
 ```rust

problems/0024.两两交换链表中的节点.md

@@ -129,6 +129,23 @@ class Solution {
 ```
 
 Python：
+```python
+class Solution:
+    def swapPairs(self, head: ListNode) -> ListNode:
+        dummy = ListNode(0) #设置一个虚拟头结点
+        dummy.next = head
+        cur = dummy
+        while cur.next and cur.next.next: 
+            tmp = cur.next #记录临时节点
+            tmp1 = cur.next.next.next #记录临时节点
+            
+            cur.next = cur.next.next          #步骤一
+            cur.next.next = tmp               #步骤二
+            cur.next.next.next = tmp1         #步骤三
+            
+            cur = cur.next.next #cur移动两位，准备下一轮交换
+        return dummy.next
+```
 
 Go：
 ```go

problems/0039.组合总和.md

@@ -237,34 +237,28 @@ public:
 
 Java：
 ```Java
+// 剪枝优化
 class Solution {
-    List<List<Integer>> lists = new ArrayList<>();
-    Deque<Integer> deque = new LinkedList<>();
-
-    public List<List<Integer>> combinationSum3(int k, int n) {
-        int[] arr = new int[]{1, 2, 3, 4, 5, 6, 7, 8, 9};
-        backTracking(arr, n, k, 0);
-        return lists;
+    public List<List<Integer>> combinationSum(int[] candidates, int target) {
+        List<List<Integer>> res = new ArrayList<>();
+        Arrays.sort(candidates); // 先进行排序
+        backtracking(res, new ArrayList<>(), candidates, target, 0, 0);
+        return res;
     }
 
-    public void backTracking(int[] arr, int n, int k, int startIndex) {
-        //如果 n 小于0，没必要继续本次递归，已经不符合要求了
-        if (n < 0) {
-            return;
-        }
-        if (deque.size() == k) {
-            if (n == 0) {
-                lists.add(new ArrayList(deque));
-            }
+    public void backtracking(List<List<Integer>> res, List<Integer> path, int[] candidates, int target, int sum, int idx) {
+        // 找到了数字和为 target 的组合
+        if (sum == target) {
+            res.add(new ArrayList<>(path));
             return;
         }
-        for (int i = startIndex; i < arr.length - (k - deque.size()) + 1; i++) {
-            deque.push(arr[i]);
-            //减去当前元素
-            n -= arr[i];
-            backTracking(arr, n, k, i + 1);
-            //恢复n
-            n += deque.pop();
+
+        for (int i = idx; i < candidates.length; i++) {
+            // 如果 sum + candidates[i] > target 就终止遍历
+            if (sum + candidates[i] > target) break;
+            path.add(candidates[i]);
+            backtracking(res, path, candidates, target, sum + candidates[i], i);
+            path.remove(path.size() - 1); // 回溯，移除路径 path 最后一个元素
         }
     }
 }

problems/0045.跳跃游戏II.md

@@ -208,6 +208,26 @@ func jump(nums []int) int {
 	}
 	return dp[len(nums)-1]
 }
+```
+
+Javascript:
+```Javascript
+var jump = function(nums) {
+    let curIndex = 0
+    let nextIndex = 0
+    let steps = 0
+    for(let i = 0; i < nums.length - 1; i++) {
+        nextIndex = Math.max(nums[i] + i, nextIndex)
+        if(i === curIndex) {
+            curIndex = nextIndex
+            steps++
+        }
+    }
+
+    return steps
+};
+```
+
 /*
 dp[i]表示从起点到当前位置的最小跳跃次数
 dp[i]=min(dp[j]+1,dp[i]) 表示从j位置用一步跳跃到当前位置，这个j位置可能有很多个，却最小一个就可以

problems/0056.合并区间.md

@@ -141,16 +141,7 @@ Java：
 class Solution {
     public int[][] merge(int[][] intervals) {
         List<int[]> res = new LinkedList<>();
-        Arrays.sort(intervals, new Comparator<int[]>() {
-            @Override
-            public int compare(int[] o1, int[] o2) {
-                if (o1[0] != o2[0]) {
-                    return Integer.compare(o1[0],o2[0]);
-                } else {
-                    return Integer.compare(o1[1],o2[1]);
-                }
-            }
-        });
+        Arrays.sort(intervals, (o1, o2) -> Integer.compare(o1[0], o2[0]));
 
         int start = intervals[0][0];
         for (int i = 1; i < intervals.length; i++) {

problems/0101.对称二叉树.md

@@ -363,6 +363,54 @@ Python：
 
 Go：
 
+```go
+/**
+ * Definition for a binary tree node.
+ * type TreeNode struct {
+ *     Val int
+ *     Left *TreeNode
+ *     Right *TreeNode
+ * }
+ */
+// 递归
+func defs(left *TreeNode, right *TreeNode) bool {
+    if left == nil && right == nil {
+        return true;
+    };
+    if left == nil || right == nil {
+        return false;
+    };
+    if left.Val != right.Val {
+        return false;
+    }
+    return defs(left.Left, right.Right) && defs(right.Left, left.Right);
+}
+func isSymmetric(root *TreeNode) bool {
+    return defs(root.Left, root.Right);
+}
+
+// 迭代
+func isSymmetric(root *TreeNode) bool {
+    var queue []*TreeNode;
+    if root != nil {
+        queue = append(queue, root.Left, root.Right);
+    }
+    for len(queue) > 0 {
+        left := queue[0];
+        right := queue[1];
+        queue = queue[2:];
+        if left == nil && right == nil {
+            continue;
+        }
+        if left == nil || right == nil || left.Val != right.Val {
+            return false;
+        };
+        queue = append(queue, left.Left, right.Right, right.Left, left.Right);
+    }
+    return true;
+}
+```
+
 
 JavaScript
 ```javascript