LeetCode96. Unique Binary Search Trees简单，简洁

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.

   1         3     3      2      1    \       /     /      / \      \     3     2     1      1   3      2    /     /       \                 \   2     1         2                 3

class Solution {public:    int numTrees(int n) {        vector<int> temp(n);        return numTrees(n, temp);    }        int numTrees(int n, vector<int>& vec)    {        if (n <= 1) return 1;        if (vec[n-1] > 0) return vec[n-1];        int count = 0;        for (int i = 1; i <= n; i++) {            count += numTrees(i-1, vec) * numTrees(n-i, vec);        }        vec[n-1] = count;        return count;    }};