Two strings of equal length can be interlaced like the teeth of a zipper to form a new word. For example, interlacing cota and rain yields croatian. The following class constructs a ZipperWord from two strings.

Your task is to complete the toString method using recursion. Hint: Concatenate the first letter from each string, follwed by a simpler ZipperWord that is made from the tails of the strings (first.substring(1) and second.substring(1)).

Complete the following file:

ZipperWord.java

public class ZipperWord { public ZipperWord(String word1, String word2) { assert word1.length() == word2.length(); first = word1; second = word2; } public String toString() { // your work here } private String first; private String second; // this method is used to check your work public static String check(String word1, String word2) { ZipperWord zippy = new ZipperWord(word1, word2); return zippy.toString(); } }