递归实现

TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
// write code here
if(t1 == nullptr || t2 == nullptr) return t1 == nullptr ? t2 : t1;
t1->val += t2->val;
t1->left = mergeTrees(t1->left, t2->left);
t1->right = mergeTrees(t1->right, t2->right);
return t1;
}