合并两个二叉树。
TreeNode* mergeTrees(TreeNode* t1, TreeNode* t2) {
if(!t1) return t2;
if(!t2) return t1;
TreeNode*node=new TreeNode(t1->val+t2->val);
node->left=mergeTrees(t1->left,t2->left);
node->right=mergeTrees(t1->right,t2->right);
return node;
}