leetcode-Combinations

Difficulty: Medium

Given two integers n and k, return all possible combinations of k numbers out of 1 ... n.

If n = 4 and k = 2, a solution is:

[ [2,4], [3,4], [2,3], [1,2], [1,3], [1,4], ] 

class Solution {
    vector<vector<int> > res;
    void helper(vector<int> &base,int n,int left,int j){
        if(left==0){
            res.push_back(base);
            return ;
        }
        
        for(int i=j;i<=n;++i){
            base.push_back(i);
            helper(base,n,left-1,i+1);
            base.pop_back();
        }
          
    }

public:
    vector<vector<int>> combine(int n, int k) {
        vector<int> base;
        helper(base,n,k,1);
        return res;
    }
};