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.

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

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