95. Unique Binary Search Trees II

[huimeich]

Given n, generate all structurally unique BST's (binary search trees) that store values 1...n.

For example, Given n = 3, your program should return all 5 unique BST's shown below.

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

[huimeich]

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
public class Solution {
    public List<TreeNode> generateTrees(int n) {
        return genTree(1, n);
    }
    public List<TreeNode> genTree(int start, int end) {
        List<TreeNode> parents = new ArrayList<TreeNode>();
        if (start > end) return parents;
        for (int i = start; i <= end; i++) {
            List<TreeNode> lefts = genTree(start, i - 1);
            List<TreeNode> rights = genTree(i + 1, end);
            for (int left = 0; left < Math.max(1,lefts.size()); left++){
                for (int right = 0; right < Math.max(1,rights.size()); right++){
                    TreeNode curr = new TreeNode(i);
                    if (lefts.size() != 0) curr.left = lefts.get(left);
                    if (rights.size() != 0) curr.right = rights.get(right);
                    parents.add(curr);
                }
            }
        }   
        return parents;
    }

}