5.3.2 Unique Binary Search Trees II

原题链接：https://oj.leetcode.com/problems/unique-binary-search-trees-ii/

我的思路：这题应该和上一题（计算有多少种BST）相似，都可以用递归来做。但是怎么从2个节点的树出发， 构建3个节点的树呢？

"We consider to use each value i as the root, and recursively generate its left subtrees that store 1…i−1 and right subtrees that store i+1…n . After that, we make each combination to combine them as a list of trees". （引用自http://www.darrensunny.me/leetcode-unique-binary-search-trees-ii/）

<span style="font-size:14px;">/** * Definition for binary tree * public class TreeNode { *     int val; *     TreeNode left; *     TreeNode right; *     TreeNode(int x) { val = x; left = null; right = null; } * } */public class Solution {    public ArrayList<TreeNode> generateTrees(int n) {        return generateTreesHelper(1, n);    }    private ArrayList<TreeNode> generateTreesHelper(int left, int right){        ArrayList<TreeNode> list = new ArrayList<TreeNode>();        if(left > right) {            list.add(null);            return list;        }        for(int i = left; i <= right; i++){            ArrayList<TreeNode> lefts = generateTreesHelper(left, i-1);            ArrayList<TreeNode> rights = generateTreesHelper(i+1, right);            //TreeNode root = new TreeNode(i);//if we put it here, will generate error below            for(int j = 0; j < lefts.size(); j++){                for(int k = 0; k < rights.size(); k++){                    TreeNode root = new TreeNode(i);                    root.left = lefts.get(j);                    root.right = rights.get(k);                    list.add(root);                }            }        }       return list;    }}</span>

Submission Result: Wrong Answ

Input:3Output:[{1,#,3,2},{1,#,3,2},{2,1,3},{3,2,#,1},{3,2,#,1}]Expected:[{1,#,2,#,3},{1,#,3,2},{2,1,3},{3,1,#,#,2},{3,2,#,1}]