![](https://img2020.cnblogs.com/blog/2083141/202007/2083141-20200715224551744-818650989.png)
思路
二叉搜索树的概念:左子树上所有结点的值小于根结点值,右子树所有结点的值大于根结点值,并且左右子树均为二叉搜索树(二叉排序树)
卡特兰数
![](https://img2020.cnblogs.com/blog/2083141/202007/2083141-20200715224552292-289074623.png)
代码
class Solution {
public int numTrees(int n) {
int[] dp = new int[n+1];
dp[0] = 1;
dp[1] = 1;
for(int i = 2; i < n + 1; i++)
for(int j = 1; j < i + 1; j++)
dp[i] += dp[j-1] * dp[i-j];
return dp[n];
}
}