#include <iostream>
#include <string>

using namespace std;

void recurse(string s, int mask = 0,string out = "")
{
  int n = s.length();
  if (out.length() == n) cout << ' ' << out<<endl;
  for (int i = 0; i < n; i++) {
    cout<<"I:"<<i<<"=>";
    unsigned bit = 1 << i;
    cout<<bit<< " -> " <<mask<<endl;
    cout<<"cond:"<<(mask & bit)<<endl;
    if (mask & bit) continue;
    cout<<out+s[i]<<endl;
    recurse(s, mask | bit, out + s[i]);
  }
}

int main()
{
  string test = "red";
  recurse(test);
  cout << endl;
  return 0;
}

I:0=>1 -> 0
cond:0
r
I:0=>1 -> 1
cond:1
I:1=>2 -> 1
cond:0
re
I:0=>1 -> 3
cond:1
I:1=>2 -> 3
cond:2
I:2=>4 -> 3
cond:0
red
 red
I:0=>1 -> 7
cond:1
I:1=>2 -> 7
cond:2
I:2=>4 -> 7     <===== here when bit is 4 and mask is 7 so condition (bit & mask) == true
cond:4              So it will continue .How i = 2 again ? and how mask becomes 1 when Mask will change when it will execute recurse(....)
I:2=>4 -> 1
cond:0
rd
I:0=>1 -> 5