leetcode 617. 合并二叉树

思路：递归求解某一棵树的左右节点，返回

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode(int x) : val(x), left(NULL), right(NULL) {}
 * };
 */
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);
        return t1;
    }
};