[LeetCode]Merge Two Binary Trees

/**
 * 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;
        } else if(t2 == NULL) {
            return t1;
        } else {
            t1->val += t2->val;
        }

        if(t1->left == NULL) {
            t1->left = t2->left;
            t2->left = NULL;
        }

        if(t1->right == NULL) {
            t1->right = t2->right;
            t2->right = NULL;
        }

        mergeTrees(t1->left, t2->left);
        mergeTrees(t1->right, t2->right);

        return t1;
    }
};

It beats 73.60% of cpp submissions.