题目描述

方法思路

这是一个动态规划问题；

class Solution {
    //Runtime: 0 ms, faster than 100.00% 
    //Memory Usage: 31.7 MB, less than 100.00%
    public int numTrees(int n) {
        if(n == 0 || n == 1) return 1;
        int[] ans = new int[n + 1];
        ans[0] = 1;
        ans[1] = 1;
        for(int i = 2; i <= n; i++){
            for(int j = 1; j <= i; j++){
                ans[i] += ans[j - 1] * ans[i - j];
            }
        }
        return ans[n];
    }
}