// Given n, how many structurally unique BST's (binary search trees) that store values 1...n?

// For example,
// Given n = 3, there are a total of 5 unique BST's.

//    1         3     3      2      1
//     \       /     /      / \      \
//      3     2     1      1   3      2
//     /     /       \                 \
//    2     1         2                 3

public class UniqueBinarySearchTree {
    public int numTrees(int n) {
        int[] dp = new int[n + 1];
        
        dp[0] = 1;
        dp[1] = 1;
        
        for(int i = 2; i <= n; i++) {
            for(int j = 1; j <= i; j++) {
                dp[i] += dp[i - j] * dp[j - 1];
            }
        }
        
        return dp[n];
    }
}