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.

        public static List<BSTNode> UniqueBinarySearchTreeII(int n)
        {
            return UniqueBinarySearchTreeIIHelper(1, n);
        }

        public static List<BSTNode> UniqueBinarySearchTreeIIHelper(int start, int end)
        {
            List<BSTNode> res = new List<BSTNode>();
            if(start > end)
            { 
                res.Add(null);
                return res;
            }
            List<BSTNode> leftChild, rightChild;
                
            for(int i=start; i<=end; i++)
            {
                leftChild = UniqueBinarySearchTreeIIHelper(start,i-1);
                rightChild = UniqueBinarySearchTreeIIHelper(i+1,end);
        
                foreach (var litem in leftChild)
                {
                    foreach (var ritem in rightChild)
                    {
                        BSTNode root = new BSTNode(i);
                        root.Left = litem;
                        root.Right = ritem;
                        res.Add(root);
                    }
                }
            }
            return res;
        }

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。