95. Unique Binary Search Trees II

Given an integern, 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

思路: 太难，不会考

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

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

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

        List<TreeNode> left,right;
        for (int i=start; i<=end; i++){
            left = genTrees(start,i-1);
            right = genTrees(i+1,end);

            for (TreeNode lnode:left){
                for (TreeNode rnode:right){
                    TreeNode root = new TreeNode(i);
                    root.left = lnode;
                    root.right = rnode;
                    list.add(root);
                }
            }
        }
        return list;
    }
}