Unique Binary Search Trees II Show result

Medium Unique Binary Search Trees II

31%

Accepted

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

Have you met this question in a real interview? 

Yes

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 of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */
public class Solution {
    /**
     * @paramn n: An integer
     * @return: A list of root
     */
    public List<TreeNode> generateTrees(int n) {
        // write your code here
        return generating(1, n);
    }
    
    private List<TreeNode> generating(int left, int right){
        
        List<TreeNode> result = new ArrayList<TreeNode>();
        if(left > right){
            result.add(null);
            return result;
        }
        if(left == right){
            result.add(new TreeNode(left));
            return result;
        }
        
        for(int i = left; i <= right; i++){
            
            for(TreeNode l : generating(left, i-1)){
                for(TreeNode r : generating(i+1, right)){
                    TreeNode node = new TreeNode(i);
                    node.left = l;
                    node.right = r;
                    result.add(node);
                }
            }
        }
        
        return result;
    }
}