LeetCode Unique Binary Search Trees

1.题目

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

2.解决方案

 class Solution {  
    public:  
        int numTrees(int n) {  
            vector<int> num;  
            num.push_back(1);