leetcode--Unique Binary Search Trees

Given n, how many structurally unique BST’s (binary search trees) that store values 1…n?

For example,
Given n = 3, there are a total of 5 unique BST’s.

class Solution {public:    int numTrees(int n) {        //递归思想，f(i)=f(i-1)*f(n-i);        //从1到n作根结点，计算种数然后累加起来。但是此解法超时        //只能放弃递归调用本函数的想法，用数组实现        int count1[n+1];        count1[0]=1;        count1[1]=1;        //构造出一个数组，第n个元素为所求值        for(int i=2;i<=n;i++)        {            count1[i]=0;   //没有初始化导致错误            for (int j=1;j<=i;j++)   //求出2到n的每个元素值            {                count1[i]+=count1[j-1] * count1[i-j]; //事先做好初始化0准备            }        }        return count1[n];    }};

下面附上递归调用函数的解法（超时）：

class Solution {public:    int numTrees(int n) {        int count1=0;        if (n<=0||n==1)        {return 1;}            for (int i=1;i<=n;i++)            { //递归思想，f(i)=f(i-1)*f(n-i);              //从1到n作根结点，计算种数然后累加起来。但是此解法超时                count1+=numTrees(i-1)*numTrees(n-i);             }            return count1;    }};