Merge pull request #104 from Joshua-Lu/patch-21

添加 0617.合并二叉树 Java版本 递归与迭代

problems/0617.合并二叉树.md

@@ -256,8 +256,10 @@ public:
 
 
 Java：
-```java
+
+```Java
 class Solution {
+    // 递归
     public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
         if (root1 == null) return root2;
         if (root2 == null) return root1;
@@ -270,6 +272,45 @@ class Solution {
 }
 ```
 
+```Java
+class Solution {
+   // 迭代
+    public TreeNode mergeTrees(TreeNode root1, TreeNode root2) {
+        if (root1 == null) {
+            return root2;
+        }
+        if (root2 == null) {
+            return root1;
+        }
+        Stack<TreeNode> stack = new Stack<>();
+        stack.push(root2);
+        stack.push(root1);
+        while (!stack.isEmpty()) {
+            TreeNode node1 = stack.pop();
+            TreeNode node2 = stack.pop();
+            node1.val += node2.val;
+            if (node2.right != null && node1.right != null) {
+                stack.push(node2.right);
+                stack.push(node1.right);
+            } else {
+                if (node1.right == null) {
+                    node1.right = node2.right;
+                }
+            }
+            if (node2.left != null && node1.left != null) {
+                stack.push(node2.left);
+                stack.push(node1.left);
+            } else {
+                if (node1.left == null) {
+                    node1.left = node2.left;
+                }
+            }
+        }
+        return root1;
+    }
+}
+```
+
 Python：