LeetCode_Combinations

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<vector<int>> result;
   vector<int>  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<vector<int> > 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：

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