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:
1 <= n <= 19
class Solution {
fun numTrees(n: Int): Int {
var result: Long = 1
for (i in 0 until n) {
result *= 2L * n - i
result /= (i + 1).toLong()
}
result /= (n + 1).toLong()
return result.toInt()
}
}