You can try the following algorithm:

1. pass through your input one digit per time
2. use a hash to count how often each digit occurs
3. once you are finished, pass through the hash (again) to see how many digits are ocuring two times
4. print yes/no

Once you have done this, think how to adapt the algorithm to handle the special case 1111....

Have Fun! Rata

PS.: it is good that you looked for an old thread! However you'll get more answers if you'd opened a new thread and just had provided a link to the old one! well, next time ;-)