95. Unique Binary Search Trees II

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LeetCode

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Problem

Given an integer n, generate all structurally unique BST’s (binary search trees) that store values 1 … n.

Example:

Input: 3
Output:
[
  [1,null,3,2],
  [3,2,null,1],
  [3,1,null,null,2],
  [2,1,3],
  [1,null,2,null,3]
]
Explanation:
The above output corresponds to the 5 unique BST's shown below:

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

Code

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode(int x) { val = x; }
 * }
 */
class Solution {
    public List<TreeNode> generateTrees(int n) {
        if(n == 0) return new ArrayList<>();
        return helper(1, n);
    }

    private List<TreeNode> helper(int start, int end){
        List<TreeNode> list = new ArrayList<>();

        if(start > end){
            list.add(null);
            return list;
        }

        for(int i = start; i <= end; i++){
            List<TreeNode> leftNodes = helper(start, i - 1);
            List<TreeNode> rightNodes = helper(i + 1, end);

            for(TreeNode left : leftNodes){
                for(TreeNode right : rightNodes){
                    TreeNode root = new TreeNode(i);
                    root.left = left;
                    root.right = right;
                    list.add(root);
                }
            }
        }

        return list;
    }
}