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

 

见上面一题

/**
 * Definition for a binary tree node.
 * 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) {
       if(n==0) return generate(1,0);
        return generate(1,n);
    }
    
    vector<TreeNode *> generate(int start,int end)
    {
        vector<TreeNode *> subtree;
        if(start>end)
        {
         subtree.push_back(NULL);
         return subtree;
         
        }
        
        for(int k=start;k<=end;k++)
        {
            vector<TreeNode *> leftsubs=generate(start,k-1);
            vector<TreeNode *> rightsubs=generate(k+1,end);
            
            for(auto i:leftsubs)
                for(auto j:rightsubs)
                {
                    TreeNode *node=new TreeNode(k);
                    node->left=i;
                    node->right=j;
                    subtree.push_back(node);
                }
        }
        return subtree;
    }
    
    
};