Merge pull request youngyangyang04#712 from ironartisan/master

添加0102.二叉树的层序遍历递归解法Python代码

Author: youngyangyang04

problems/0102.二叉树的层序遍历.md

@@ -87,7 +87,9 @@ public:
 
 python代码：
 
+
 ```python3
+
 class Solution:
     """二叉树层序遍历迭代解法"""
 
@@ -113,7 +115,20 @@ class Solution:
 
         return results
 ```
-
+```python
+# 递归法
+class Solution:
+    def levelOrder(self, root: TreeNode) -> List[List[int]]:
+        res = []
+        def helper(root, depth):
+            if not root: return []
+            if len(res) == depth: res.append([]) # start the current depth
+            res[depth].append(root.val) # fulfil the current depth
+            if  root.left: helper(root.left, depth + 1) # process child nodes for the next depth
+            if  root.right: helper(root.right, depth + 1)
+        helper(root, 0)
+        return res
+```
 java: 
 
 ```Java

problems/0222.完全二叉树的节点个数.md

@@ -204,7 +204,27 @@ class Solution {
     }
 }
 ```
-
+```java
+class Solution {
+    // 迭代法
+    public int countNodes(TreeNode root) {
+        if (root == null) return 0;
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+        int result = 0;
+        while (!queue.isEmpty()) {
+            int size = queue.size();
+            while (size -- > 0) {
+                TreeNode cur = queue.poll();
+                result++;
+                if (cur.left != null) queue.offer(cur.left);
+                if (cur.right != null) queue.offer(cur.right);
+            }
+        }
+        return result;
+    }
+}
+```
 ```java
 class Solution {
     /**

problems/0404.左叶子之和.md

@@ -201,7 +201,31 @@ class Solution {
     }
 }
 ```
-
+```java
+// 层序遍历迭代法
+class Solution {
+    public int sumOfLeftLeaves(TreeNode root) {
+        int sum = 0;
+        if (root == null) return 0;
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root);
+        while (!queue.isEmpty()) {
+            int size = queue.size();
+            while (size -- > 0) {
+                TreeNode node = queue.poll();
+                if (node.left != null) { // 左节点不为空
+                    queue.offer(node.left);
+                    if (node.left.left == null && node.left.right == null){ // 左叶子节点
+                        sum += node.left.val;
+                    }
+                }
+                if (node.right != null) queue.offer(node.right);
+            }
+        }
+        return sum;
+    }
+}
+```
 
 
 ## Python