[LeetCode] 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         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.

Solution:

/**
 * 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 *> generate(int start, int end)
    {
      vector<TreeNode *> ans;
      if(start > end)
      {
        ans.push_back(NULL);
        return ans;
      }
      for(int i = start;i <= end;i++)
      {
        vector<TreeNode *> left = generate(start, i - 1);
        vector<TreeNode *> right = generate(i + 1, end);

        for(int j = 0;j < left.size();j++)
        {
          for(int k = 0;k < right.size();k++)
          {
            TreeNode *cur = new TreeNode(i);
            cur -> left = left[j];
            cur -> right = right[k];
            ans.push_back(cur);
          }
        }
      }
      return ans;
    }

    vector<TreeNode *> generateTrees(int n) {
      return generate(1, n);
    }
};