Select two numbers between 1 and 9998, inclusive, which have the same exact group of non-zero digits, but are not the same number. For example, you could use 1234 and 4321, or 91 and 901. Now, subtract the smaller of the two numbers from the larger. Finally, pick one of the non-zero digits in the result, and remove the digit from the number. If the resulting number is less than 100, prepend enough zeros so that it has 3 digits. It turns out, that given the remaining 3 digits, one can always determine the digit removed. (See examples for clarification)
You will be given a String leftOver, which contains the three digits left over after the above algorithm is run. You must return the digit which was removed.