Answer: '' (An empty string)
Explanation:
Normally, when we are getting a substring, we will start from the character at index m and go until, but exclude the character at index n.
If m is greater than n, then we cannot get any substring as we would have to go backwards when we need to go forwards. If m=n, then we still cannot get any characters as we have to exclude the character at index n, making us have to exclude the character at index m as well.
Hence, for both situations the value would be an empty string, or a string with length 0.
