Update 0617.合并二叉树.md - 增加了python3版本的迭代解法

problems/0617.合并二叉树.md

@@ -319,7 +319,7 @@ Python：
 #         self.val = val
 #         self.left = left
 #         self.right = right
-//递归法*前序遍历
+# 递归法*前序遍历
 class Solution:
     def mergeTrees(self, root1: TreeNode, root2: TreeNode) -> TreeNode:
         if not root1: return root2  // 如果t1为空，合并之后就应该是t2
@@ -328,6 +328,32 @@ class Solution:
         root1.left = self.mergeTrees(root1.left , root2.left)  //左
         root1.right = self.mergeTrees(root1.right , root2.right)  //右
         return root1  //root1修改了结构和数值
+
+# 迭代法-覆盖原来的树      
+class Solution:
+    def mergeTrees(self, root1: TreeNode, root2: TreeNode) -> TreeNode:
+        if not root1: return root2
+        if not root2: return root1
+        # 迭代,将树2覆盖到树1
+        queue1 = [root1]
+        queue2 = [root2]
+        root = root1
+        while queue1 and queue2:
+            root1 = queue1.pop(0)
+            root2 = queue2.pop(0)
+            root1.val += root2.val
+            if not root1.left: # 如果树1左儿子不存在，则覆盖后树1的左儿子为树2的左儿子
+                root1.left = root2.left
+            elif root1.left and root2.left:
+                queue1.append(root1.left)
+                queue2.append(root2.left)
+
+            if not root1.right: # 同理，处理右儿子
+                root1.right = root2.right
+            elif root1.right and root2.right:
+                queue1.append(root1.right)
+                queue2.append(root2.right)
+        return root
 ```
 
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