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;
    }
};