LeetCode-Unique Binary Search Trees

同上一道返回bst树的题目的思路，

求数目的话更加简单；

class Solution {
public:
    int numTrees(int n) {
        // Start typing your C/C++ solution below
        // DO NOT write int main() function
        if (n <= 0) {
            return 0;
        }
        int count = 0;
        getNum(count, 1, n + 1);
        return count;
    }
    void getNum(int &count, int l, int r) {
        if (l == r) {
            count = 1;
            return;
        }
        for (int i = l; i < r; ++i) {
            int left = 0;
            getNum(left, l, i);
            int right = 0;
            getNum(right, i + 1, r);
            count += left * right;
        }
    }
};