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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
# @param {Integer} n
# @return {Integer}
def num_trees(n)
result = 1
for i in 0...n
result *= (2 * n - i)
result /= (i + 1)
end
result /= (n + 1)
result.to_i
end