TavasAndSaddas.java

private int findIndex(String S){
  int N = S.length();
  int ans = (1<<N)-2;
  int pos=0,i=N-1;
  while(i>=0) {
    if(S.charAt(i)=='7') {
      ans += (1<<pos);
    }
    i--;
    pos++;
  }
  return (ans+1);
}