Leetcode 95 Unique Binary Search Trees II

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

Solution 1: Divide and conquer. In order to generate 1...n, we can choose any of them to be root, let us say i, root.left is from list of 0...i-1 and root.right is from list of i+1...n. Code as below:

 public class Solution {  
   public List<TreeNode> generateTrees(int n) {  
     return generate(1,n);  
   }  
   private List<TreeNode> generate(int lo, int hi) {  
     List<TreeNode> res=new ArrayList<>();  
     if (lo>hi) res.add(null);  
     else {  
       for (int i=lo; i<=hi; i++) {  
         for (TreeNode l: generate(lo,i-1)) {  
           for (TreeNode r: generate(i+1,hi)) {  
             TreeNode x=new TreeNode(i);  
             x.left=l;  
             x.right=r;  
             res.add(x);  
           }  
         }  
       }  
     }  
     return res;  
   }  
 }