Iterative Subsequence

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


int main(){
    string str = "abcd";
    int N = 1<<str.size();
    for (int i=0; i<N; i++) {
        for (int j=0; j<str.size(); j++) {
            if ((i>>j) & 1) {
                cout<<str[j];
            }
        }
        cout<<endl;
    }
}