[Medium] Unique Binary Search Trees

问题：
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
解法：

源码：

class Solution {public:    int numTrees(int n) {        int f[n];        f[0] = 1;        f[1] = 1;        for (int i = 2; i <= n; ++i) {            f[i] = 0;            for (int j = 0; j <= i-1; ++j) {                f[i] += f[j] * f[i-1-j];            }        }        return f[n];    }};