class Solution {
public:int numTrees(int n) {if (n==0) {return 1;}vector<int> dp(n+1, 0);dp[0] = 1;dp[1] = 0;for (int i = 1; i <= n; i++) {for (int j = 0; j < i; j++) {dp[i] += dp[j] * dp[i - 1 - j];}}return dp[n];}
};