#include <iostream>
#include<set>
using namespace std;

void solve(string str,int k,string current,int start,vector<string> &comb){
   if(current.size()==k){
       comb.push_back(current);
       return;
   }
   if(start==str.size()){
       return;
   }
   solve(str,k,current+str[start],start+1,comb);
   solve(str,k,current,start+1,comb);
}
int main() {
    string str="ABCDE";
    int k=2;
    vector<string> comb;
    solve(str,k,"",0,comb);
    
    return 0;
}