Unique Binary Search Trees II

Unique Binary Search Trees II.java

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



/**
 * Definition for binary tree
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; left = null; right = null; }
 * }
 */
public class Solution {
    public ArrayList<TreeNode> generateTrees(int n) {
        // Start typing your Java solution below
        // DO NOT write main() function
     
     ArrayList<TreeNode> result=generateBST(1,n);
     
     return result;
    }
    public ArrayList<TreeNode> generateBST(int start, int end){
        ArrayList<TreeNode> subTree=new ArrayList<TreeNode>();
        
        if (start>end){
            subTree.add(null);
        }
        else if  (start==end){
            
            subTree.add(new TreeNode(start));
           
            
        }
        else{
             for (int i=start; i<=end; i++){
               // divied and conquer.
               // get collection of all left trees and right trees
               // then construct them together
                ArrayList<TreeNode> leftTree=generateBST(start, i-1);
                ArrayList<TreeNode> rightTree=generateBST(i+1, end);
                
                for (TreeNode leftNode:leftTree){
                    for(TreeNode rightNode:rightTree){
                        TreeNode root=new TreeNode(i);
                        root.left=leftNode;
                        root.right=rightNode;
                                   
                        subTree.add(root);
                      }
                 }
                       
             }
             
        }
        return subTree;
    }
}