add 617. Merge Two Binary Trees 🍺

solutions/617. Merge Two Binary Trees.md

@@ -0,0 +1,24 @@
+# [617. Merge Two Binary Trees](https://leetcode.com/problems/merge-two-binary-trees/)
+
+# 思路
+
+合并两个二叉树。就是一个很简单的递归：
+* 若`t1`和`t2`任意一个为空，那么直接返回另一个即可（递归出口）；
+* 否则，将`t1`的值加上`t2`的值，然后递归合并`t1`和`t2`的左子树以及`t1`和`t2`的右子树。
+
+# C++
+``` C++
+class Solution {
+public:
+    TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
+        if(t1 == NULL) return t2;
+        if(t2 == NULL) return t1;
+        
+        t1 -> val += t2 -> val;
+        t1 -> left = mergeTrees(t1 -> left, t2 -> left);
+        t1 -> right = mergeTrees(t1 -> right, t2 -> right);
+        delete t2;
+        return t1;
+    }
+};
+```