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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:
1 <= n <= 19
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;
}
}