problem 2

.java

String stringReformatting(String s, int k) {

    String[] stringParts = s.split("-");
    String newString = "";
    String reformattedString = "";
    int startLocation = 0;
    for (String stringPart : stringParts) {
        newString = newString + stringPart;
    }
    int firstSegmentSize = newString.length() % k;
    
    reformattedString = newString.substring(0, firstSegmentSize);
    startLocation += firstSegmentSize;
    for (int i = startLocation; i < newString.length(); i+=k) {
        reformattedString = reformattedString + "-" + 
            newString.substring(i, i+k);
    }
    if (reformattedString.startsWith("-")) {
        reformattedString = reformattedString.substring(1);
    }
    return reformattedString;
}

algorithm looks like it works.

This does it in O(2n), potentially 2 full passes over the string (1 to remove hyphens, 1 to rebuild). I can think of a way it can be done in O(n), 1 pass over the string.