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],]

class Solution {private:   vector
     
      
       > result;   vector
       
          a ; public:    void findResult(int n, int k, int num,int start )   {      if(num == k)       {          result.push_back(a);          return;       }       for(int i = start; i <= n; i++)       {                  a[num] = i;          findResult( n,  k,  num+1, i+1);               }   }    vector
        
         
           > combine(int n, int k) {        // Start typing your C/C++ solution below        // DO NOT write int main() function                a.resize(k);       result.clear();       findResult( n, k, 0, 1) ;       return result;         }};
         
        
       
      
     

 重写后：

//Combinationsclass Solution {public:    void DFS(int n, int start,int k, vector
     
       &ans){        if(ans.size() == k){            res.push_back(ans);            return;        }        for(int i = start; i <= n; i++)        {            ans.push_back(i);            DFS(n, i+1, k, ans);            ans.pop_back();        }    }    vector
      
       
         > combine(int n, int k) {        // Start typing your C/C++ solution below        // DO NOT write int main() function        res.clear();        vector
        
          ans;        DFS(n, 1, k, ans);        return res;            }private:    vector
         
          
           > res;};