public class Solution {
    public int numTrees(int n) {
        int[] dp = new int[n+1];
        
        dp[0] = 1;
        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];
    }
}
