(Hackerrank)Binary code problem

/*Binary Code
 
You are given a sequence of bits (a binary string) of length N. You need to generate a list of binary strings of length N starting from the given binary string. There can be no repetitions in the list, and any two consecutive binary strings in the list should differ by only one bit.

For example, assume you are given the binary string 0010. binary strings 0010 and 0110 differ only by one bit; 0010 and 0111 differ by two bits; 0010 and 0010 are the same; 0200 is not a binary string; 100 is not a binary string of length 4, while 0100 is a binary string of length 4.

This problem has partial scores. If your list is valid, then your score for that test case is {L\over 2^N}S, where L is the number of binary strings in your list and S is the allocated score for the test case.

Input Format
The input will contain a non empty string of 0’s and 1’s indicating the initial binary string.

Constraints
1 ≤ N ≤ 16  
Limits
Time Limit: 1s
Memory Limit: 256MB
Output Format
First line of the output should contain a single integer L, the number of strings in your list. L lines should follow, the list of values.

Sample Input
0010

Sample Output 0
6
0010
1010
1000
1100
0100
0000
*/

//Solution.

#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <sstream>
#include <map>
#include <set>
#include <cmath>
#include <cstring>
#include <cstdio>

using namespace std;

string s;

void change(int n) {
    if (n >= s.size()) {
        cout << s << endl;
        return;
    }

    change(n + 1);
    s[n] = s[n] == '0' ? '1' : '0';
    change(n + 1);
}

int main() {
    cin >> s;

    cout << (1 << s.size()) << endl;

    change(0);

    return 0;
}