public class Solution {
    public int numTrees(int n) {
        if(n<2) return n;
        int dp[] = new int[n+1];
        dp[0] = 1;
        dp[1] = 1;
        for(int i=2; i<=n; i++) {
            for(int j=0; j<i/2; j++) {
                dp[i] += 2*dp[j]*dp[i-1-j];
            }
            if(i % 2 != 0)  dp[i] += dp[i/2] * dp[i/2];
        }
        return dp[n];
    }
}