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];
    }