Combinations

来源：Leetcode

原帖：http://oj.leetcode.com/problems/combinations/

题目：
Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.
For example,
If n = 4 and k = 2, a solution is:
 [
    [2,4],
    [3,4],
    [2,3],
    [1,2],
    [1,3],
    [1,4],
 ]

代码：

 class Solution {  
 public:  
   vector<vector<int> > combine(int n, int k) {  
     vector<vector<int> > res;  
     vector<int> onecom;  
     combineHelper(n, k, 1, onecom, res);  
     return res;  
   }  
     
   void combineHelper(int n, int k, int start, vector<int> &onecom, vector<vector<int> > &res) {  
     int m = onecom.size();  
     if (m == k) {  
       res.push_back(onecom);  
       return;  
     }  
     for (int i = start; i <= n - (k - m) + 1; ++i) { // index = n-(k-m);  
       onecom.push_back(i);  
       combineHelper(n, k, i + 1, onecom, res);  
       onecom.pop_back();  
     }  
   }  
 };