LeetCode-in-All

96. Unique Binary Search Trees

Medium

Given an integer n, return the number of structurally unique BST’s (binary search trees) which has exactly n nodes of unique values from 1 to n.

Example 1:

Input: n = 3

Output: 5

Example 2:

Input: n = 1

Output: 1

Constraints:

Solution

public class Solution {
    public int NumTrees(int n) {
        long result = 1;
        for (int i = 0; i < n; i++) {
            result *= 2L * n - i;
            result /= i + 1;
        }
        result /= n + 1;
        return (int) result;
    }
}