Unique Binary Search Trees

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

• Time: O(n)
• Space: O(1)

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 = 0; j <= i; j++) {
            dp[i] += dp[j] * dp[i - j - 1];
        }
    }
    return dp[n];
}