增加 0617.合并二叉树 go版 （增加前序遍历简洁版解题）

增加前序遍历简洁版解题

problems/0617.合并二叉树.md

@@ -368,6 +368,20 @@ func mergeTrees(t1 *TreeNode, t2 *TreeNode) *TreeNode {
         Right: mergeTrees(t1.Right,t2.Right)}
     return root
 }
+
+// 前序遍历简洁版
+func mergeTrees(root1 *TreeNode, root2 *TreeNode) *TreeNode {
+    if root1 == nil {
+        return root2
+    }
+    if root2 == nil {
+        return root1
+    }
+    root1.Val += root2.Val
+    root1.Left = mergeTrees(root1.Left, root2.Left)
+    root1.Right = mergeTrees(root1.Right, root2.Right)
+    return root1
+}
 ```
 
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