LeetCode 95. Unique Binary Search Trees II

题目如下：

Given an integer 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

思路： 

递归产生子树，感觉这题很经典。 容易出错的地方有:

(1) if(n<=0) 要判断，否则，n＝0的测试结果是 [[]] ,而正确应该返回 []  

(2) if (s > e) {res.add(null);} 这里不能直接{return null;}

本题代码：

import java.util.ArrayList;
import java.util.List;

/**
 * Created by yuanxu on 17/4/13.
 */
public class DP95 {

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

    private static List<TreeNode> generateSubtrees(int s, int e) {
        List<TreeNode> res = new ArrayList<TreeNode>();
        if (s > e) { res.add(null); }

        for (int i=s; i<=e; ++i) {
            List<TreeNode> leftTrees = generateSubtrees(s, i-1);
            List<TreeNode> rightTrees = generateSubtrees(i+1, e);

            for (TreeNode left : leftTrees) {
                for (TreeNode right : rightTrees) {
                    TreeNode root = new TreeNode(i);
                    root.left = left;
                    root.right = right;
                    res.add(root);
                }
            }
        }
        return res;
    }

    static class TreeNode {
        int val;
        TreeNode left;
        TreeNode right;
        TreeNode(int x) { val = x; }
    }

    /**
     * @test
     */
    public static void main(String args[]) {
        int n = 0;
        System.out.println(generateTrees(n).get(0).val);
    }

}

ref ： https://discuss.leetcode.com/topic/3079/a-simple-recursive-solution