Leetcode96. 不同的二叉搜索树

题目传送：​​https://leetcode.cn/problems/unique-binary-search-trees/submissions/​​

运行效率

代码如下：

public static int numTrees(int n) {
        int num = generateNum(1, n);
        return num;
    }

    public static int generateNum(int start, int end) {
        int count = 0;
        //处理边界情况
        if (start == end) {
            return 1;
        }
        //1、先确定二叉搜索树的根节点
        for (int i = start; i <= end; i++) {
            int leftNum = 1;
            int rightNum = 1;
            if (i - 1 >= start) {
            //左边二叉树的所有可能情况数
                leftNum = generateNum(start, i - 1);
            }
             //右边二叉树的所有可能情况数
            if (i + 1 <= end) {
                rightNum = generateNum(i + 1, end);
            }
            count +=leftNum*rightNum;
        }
        return count;
    }