[LeetCode]96. Unique Binary Search Trees（DP，卡特兰数）

题目链接：https://leetcode.com/problems/unique-binary-search-trees/#/description

题意：求二叉树种类

卡特兰数，解释一下：n个点取第i个点为父亲，左右儿子可能有C(0)*C(n-1)+C(1)*C(n-2)+...+C(「i/2)*(i/2」)种。这样就可以dp了。

class Solution {
public:
    int s[123] = {0,
                                1,
                                2,
                                5,
                                14,
                                42,
                                132,
                                429,
                                1430,
                                4862,
                                16796,
                                58786,
                                208012,
                                742900,
                                2674440,
                                9694845,
                                35357670,
                                129644790,
                                477638700,
                                1767263190};

    int numTrees(int n) {
        return s[n];
  }
};