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/**
 * 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 {
            TreeNode* t3 = new TreeNode(t1->val+t2->val);
            t3->left = mergeTrees(t1->left, t2->left);
            t3->right = mergeTrees(t1->right, t2->right);
            return t3;
        }
    }
};