添加0617.合并二叉树.md队列迭代Java解法

Author: ironartisan

problems/0617.合并二叉树.md

@@ -274,7 +274,7 @@ class Solution {
 
 ```Java
 class Solution {
-   // 迭代
+    // 使用栈迭代
     public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
         if (root1 == null) {
             return root2;
@@ -310,6 +310,43 @@ class Solution {
     }
 }
 ```
+```java
+class Solution {
+    // 使用队列迭代
+    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
+        if (root1 == null) return root2;
+        if (root2 ==null) return root1;
+        Queue<TreeNode> queue = new LinkedList<>();
+        queue.offer(root1);
+        queue.offer(root2);
+        while (!queue.isEmpty()) {
+            TreeNode node1 = queue.poll();
+            TreeNode node2 = queue.poll();
+            // 此时两个节点一定不为空，val相加
+            node1.val = node1.val + node2.val;
+            // 如果两棵树左节点都不为空，加入队列
+            if (node1.left != null && node2.left != null) {
+                queue.offer(node1.left);
+                queue.offer(node2.left);
+            }
+            // 如果两棵树右节点都不为空，加入队列
+            if (node1.right != null && node2.right != null) {
+                queue.offer(node1.right);
+                queue.offer(node2.right);
+            }
+            // 若node1的左节点为空，直接赋值
+            if (node1.left == null && node2.left != null) {
+                node1.left = node2.left;
+            }
+            // 若node2的左节点为空，直接赋值
+            if (node1.right == null && node2.right != null) {
+                node1.right = node2.right;
+            }
+        }
+        return root1;
+    }
+}
+```
 
 ## Python