LeetCode | Unique Binary Search Trees II

題目：

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         3     3      2      1    \       /     /      / \      \     3     2     1      1   3      2    /     /       \                 \   2     1         2                 3

confused what "{1,#,2,3}" means? > read more on how binary tree is serialized on OJ.
思路： 類似 http://blog.csdn.net/lanxu_yy/article/details/17504123，找到一個數作為根結點，剩餘的數分別劃入左子樹或者右子樹。
代碼：

/** * Definition for binary tree * struct TreeNode { *     int val; *     TreeNode *left; *     TreeNode *right; *     TreeNode(int x) : val(x), left(NULL), right(NULL) {} * }; */class Solution {public:    vector<TreeNode *> generateTrees(int n) {       return createTree(1,n);    }        vector<TreeNode *> createTree(int start, int end)    {        vector<TreeNode *> results;        if(start>end)        {            results.push_back(NULL);            return results;        }                for(int k=start;k<=end;k++)        {            vector<TreeNode *> left = createTree(start,k-1);            vector<TreeNode *> right = createTree(k+1,end);            for(int i=0;i<left.size();i++)            {                for(int j=0;j<right.size();j++)                {                    TreeNode * root = new TreeNode(k);                    root->left = left[i];                    root->right = right[j];                    results.push_back(root);                }            }        }        return results;    }};