Update 0404.左叶子之和.md

problems/0404.左叶子之和.md

@@ -247,8 +247,7 @@ class Solution {
 
 
 ### Python 
-
+递归
-**递归后序遍历**
 ```python
 # Definition for a binary tree node.
 # class TreeNode:
@@ -257,47 +256,64 @@ class Solution {
 #         self.left = left
 #         self.right = right
 class Solution:
-    def sumOfLeftLeaves(self, root: Optional[TreeNode]) -> int:
+    def sumOfLeftLeaves(self, root):
-        if not root:
+        if root is None:
+            return 0
+        if root.left is None and root.right is None:
             return 0
         
-        # 检查根节点的左子节点是否为叶节点
+        leftValue = self.sumOfLeftLeaves(root.left)  # 左
-        if root.left and not root.left.left and not root.left.right:
+        if root.left and not root.left.left and not root.left.right:  # 左子树是左叶子的情况
-            left_val = root.left.val
+            leftValue = root.left.val
-        else:
-            left_val = self.sumOfLeftLeaves(root.left)
             
-        # 递归地计算右子树左叶节点的和
+        rightValue = self.sumOfLeftLeaves(root.right)  # 右
-        right_val = self.sumOfLeftLeaves(root.right)
+
-        
+        sum_val = leftValue + rightValue  # 中
-        return left_val + right_val
+        return sum_val
+```
+递归精简版
+
+```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
+class Solution:
+    def sumOfLeftLeaves(self, root):
+        if root is None:
+            return 0
+        leftValue = 0
+        if root.left is not None and root.left.left is None and root.left.right is None:
+            leftValue = root.left.val
+        return leftValue + self.sumOfLeftLeaves(root.left) + self.sumOfLeftLeaves(root.right)
 ```
 
-**迭代**
+迭代法
 ```python
+# Definition for a binary tree node.
+# class TreeNode:
+#     def __init__(self, val=0, left=None, right=None):
+#         self.val = val
+#         self.left = left
+#         self.right = right
 class Solution:
-    def sumOfLeftLeaves(self, root: TreeNode) -> int:
+    def sumOfLeftLeaves(self, root):
-        """
+        if root is None:
-        Idea: Each time check current node's left node. 
+            return 0
-              If current node don't have one, skip it. 
+        st = [root]
-        """
+        result = 0
-        stack = []
+        while st:
-        if root: 
+            node = st.pop()
-            stack.append(root)
+            if node.left and node.left.left is None and node.left.right is None:
-        res = 0
+                result += node.left.val
-        
+            if node.right:
-        while stack: 
+                st.append(node.right)
-            # 每次都把当前节点的左节点加进去. 
+            if node.left:
-            cur_node = stack.pop()
+                st.append(node.left)
-            if cur_node.left and not cur_node.left.left and not cur_node.left.right: 
+        return result
-                res += cur_node.left.val
+
-                
-            if cur_node.left: 
-                stack.append(cur_node.left)
-            if cur_node.right: 
-                stack.append(cur_node.right)
-                
-        return res
 ```
 
 ### Go