LeetCode Online Judge 题目C# 练习 - Unique Binary Search Trees II-优快云博客

本文介绍了一种使用递归方法生成所有可能的独特二叉搜索树的方法。对于给定的整数n，该算法能生成所有结构上唯一的二叉搜索树，这些树能够存储从1到n的值。以n为3为例，程序将返回5种不同的二叉搜索树。

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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         public static List<BSTNode> UniqueBinarySearchTreeII(int n)
 2         {
 3             return UniqueBinarySearchTreeIIHelper(1, n);
 4         }
 5 
 6         public static List<BSTNode> UniqueBinarySearchTreeIIHelper(int start, int end)
 7         {
 8             List<BSTNode> res = new List<BSTNode>();
 9             if(start > end)
10             { 
11                 res.Add(null);
12                 return res;
13             }
14             List<BSTNode> leftChild, rightChild;
15                 
16             for(int i=start; i<=end; i++)
17             {
18                 leftChild = UniqueBinarySearchTreeIIHelper(start,i-1);
19                 rightChild = UniqueBinarySearchTreeIIHelper(i+1,end);
20         
21                 foreach (var litem in leftChild)
22                 {
23                     foreach (var ritem in rightChild)
24                     {
25                         BSTNode root = new BSTNode(i);
26                         root.Left = litem;
27                         root.Right = ritem;
28                         res.Add(root);
29                     }
30                 }
31             }
32             return res;
33         }