# [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;
    }
};
```