Leetcode---Unique Binary Search Trees

Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

很简单，就是求卡特兰数，直接公式求

    int numTrees(int n) {        vector<int> num(n+1);num[0]=1;num[1]=1;num[2]=2;for(int j=3;j<=n;j++){for(int i=0;i<j;i++){num[j]+=num[j-i-1]*num[i];}}num[0]=0;return num[n];    }