Merge branch 'youngyangyang04:master' into master

Author: MengyiG

problems/0015.三数之和.md

@@ -403,6 +403,7 @@ class Solution:
 ```
 
 ### Go：
+（版本一） 双指针
 
 ```Go
 func threeSum(nums []int) [][]int {
@@ -442,6 +443,42 @@ func threeSum(nums []int) [][]int {
 	return res
 }
 ```
+（版本二） 哈希解法
+
+```Go
+func threeSum(nums []int) [][]int {
+    res := make([][]int, 0)
+    sort.Ints(nums)
+    // 找出a + b + c = 0
+    // a = nums[i], b = nums[j], c = -(a + b)
+    for i := 0; i < len(nums); i++ {
+        // 排序之后如果第一个元素已经大于零，那么不可能凑成三元组
+        if nums[i] > 0 {
+            break
+        }
+        // 三元组元素a去重
+        if i > 0 && nums[i] == nums[i-1] {
+            continue
+        }
+        set := make(map[int]struct{})
+        for j := i + 1; j < len(nums); j++ {
+            // 三元组元素b去重
+            if j > i + 2 && nums[j] == nums[j-1] && nums[j-1] == nums[j-2] {
+                continue
+            }
+            c := -nums[i] - nums[j]
+            if _, ok := set[c]; ok {
+                res = append(res, []int{nums[i], nums[j], c})
+                // 三元组元素c去重
+                delete(set, c)
+            } else {
+                set[nums[j]] = struct{}{}
+            }
+        }
+    }
+    return res
+}
+```
 
 ### JavaScript: 
 

problems/0102.二叉树的层序遍历.md

@@ -201,7 +201,7 @@ class Solution:
         return result
 ```
 ```python
-# 递归法
+#递归法
 # Definition for a binary tree node.
 # class TreeNode:
 #     def __init__(self, val=0, left=None, right=None):
@@ -210,18 +210,24 @@ class Solution:
 #         self.right = right
 class Solution:
     def levelOrder(self, root: Optional[TreeNode]) -> List[List[int]]:
+        if not root:
+            return []
+
         levels = []
-        self.helper(root, 0, levels)
+
+        def traverse(node, level):
+            if not node:
+                return
+
+            if len(levels) == level:
+                levels.append([])
+
+            levels[level].append(node.val)
+            traverse(node.left, level + 1)
+            traverse(node.right, level + 1)
+
+        traverse(root, 0)
         return levels
-    
-    def helper(self, node, level, levels):
-        if not node:
-            return
-        if len(levels) == level:
-            levels.append([])
-        levels[level].append(node.val)
-        self.helper(node.left, level + 1, levels)
-        self.helper(node.right, level + 1, levels)
 
 ```