[LC] 95. Unique Binary Search Trees II

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

/**
 * 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 genList(1, n);
    }
    
    private List<TreeNode> genList(int start, int end) {
        List<TreeNode> list = new ArrayList<>();
        if (start > end) {
            list.add(null);
        }
        for (int i = start; i <= end; i++) {
            List<TreeNode> leftNodes = genList(start, i - 1);
            List<TreeNode> rightNodes = genList(i + 1, end);
            for (TreeNode leftNode: leftNodes) {
                for(TreeNode rightNode: rightNodes) {
                    TreeNode curNode = new TreeNode(i);
                    curNode.left = leftNode;
                    curNode.right = rightNode;
                    list.add(curNode);
                }
            }
        }
        return list;
    }
}