题目描述:

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],]

解题思路:

1 class Solution { 2 public: 3     vector
     
      
       > combine(int n, int k) { 4         vector
       
        
         > result; 5         vector
         
           elem; 6          7         combine(n, k, 1, result, elem); 8         return result; 9     }10 private:11     void combine(int n, int k, int cur, vector
          
           
            > &result, vector
            
              &elem) {12 if (elem.size() == k) {13 result.push_back(elem);14 return;15 }16 17 for (int i = cur; i <= n; ++i) {18 elem.push_back(i);19 combine(n, k, i + 1, result, elem);20 elem.pop_back();21 }22 }23 };