[LeetCode] 96. 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

关键点：卡特兰数

    int numTrees(int n) {        long h = 1;        for (int i = 2; i <= n; i++)            h = h * (4 * i - 2) / (i + 1);        return h;    }