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

class Solution {

    public List<TreeNode> generateTrees(int n) {
        if(n == 0) return new ArrayList<>();
        return generate(1, n);
    }

    public List<TreeNode> generate(int l, int r) {
        List<TreeNode> result = new ArrayList<TreeNode>();
        if(l > r) result.add(null);
        for(int i = l; i <= r; i++) {
            List<TreeNode> left = generate(l, i-1), right = generate(i+1, r);
            for(TreeNode ln : left) {
                for(TreeNode rn : right) {
                    TreeNode root = new TreeNode(i);
                    root.left = ln;
                    root.right = rn;
                    result.add(root);
                }
            }
        }
        return result;
    }
  }