Write a recursive method substring that accepts as parameters a string, a start index, and an ending index, and returns a specified substring of the string.
Your method should return the substring that begins at the start index and that extends to the character just before the ending index.
For example, the call of substring("hamburger", 4, 8) should return "urge".
The method should throw an IllegalArgumentException if the start index is negative or if the ending index is greater than the length of the string or if the start index is greater than the ending index.
The method should return an empty string if the two indexes are equal.
Constraints:
In implementing this method, you are restricted to the following string methods:
charAt, equals, and length.
Do not construct any structured objects other than strings (no list, StringBuilder,
