1. 推导过程

2.代码

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 = 1;j <= i;j++){
                dp[i] += dp[j-1] * dp[i-j];
            }
        }
        return dp[n];
    }
}