动态规划

public class Solution {
    public int numTrees (int 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 ; j++){
                dp[i] += dp[j] * dp[i - j - 1];
            }
        }

        return dp[n];
    }
}