HackerHeap

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

Example:

Input: 3
Output: 5
Explanation:
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

Solution:

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