LeetCode - combinations (seeking an array containing a number of combinations of all)

Disclaimer: This article is written self-study, pointed out that if the error also hope, thank you ^ - ^ https://blog.csdn.net/weixin_43871369/article/details/91439497

Title Description

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],
]

//dfs爆搜所有结果
class Solution {
public:
    vector<vector<int> > combine(int n, int k) {
        vector<vector<int> >ans;
        if(n<k) return ans;
        vector<int>vec;
        for(int i=1;i<=n-k+1;++i)
        {
            vec.push_back(i);
            dfs(ans,vec,i,n,k,1);
            vec.pop_back();
        }
        return ans;    
    }
 private:
    void dfs(vector<vector<int> >&res,vector<int>sub,int index,int n,int k,int cnt)
    {
         if(cnt>=k){
             res.push_back(sub);
             return ;
         }
        for(int i=index+1;i<=n;++i)
        {
            sub.push_back(i);
            dfs(res,sub,i,n,k,cnt+1);
            sub.pop_back();
        }
        return ;
    }
};