+1计算1-n有多个不同的二叉树

public class Solution {    public int numTrees(int n) {        if (n <= 0)            return 0;        if (n == 1)            return 1;        if (n == 2)            return 2;        int[] rs = new int[n + 1];        rs[0] = 1;        rs[1] = 1;        rs[2] = 2;        for (int i = 3; i < n + 1; i++) {            int tmp = 0;            for (int j = 1; j <= i; j++ ) {                tmp += rs[j - 1]*rs[i - j];            }            rs[i] = tmp;        }                return rs[n];    }}

差一点就推出来了，差一点没想到。