LeetCode Online Judge 题目C# 练习 - 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         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         }

代码分析：

　　递归，分别求出左边，右边的subtree root，在二重循环把他们加到root上。