Given a string str, two int values, and a boolean value, write a method that works similar to the String.substring method if the given boolean value is true. e.g. substring("hello!", 0, 2, true) --> "he". If the boolean value is false, return a new string starting from the first given int value all the way to the end of the string. e.g. substring("hello!", 0, 2, false) --> "hello!".