How do you find the alphabetically last letter of a string using recursion (no loops!) and without u-CodePudding

Got something for you all.

As the title of the problem suggests, I am trying to implement a non-array, non-looping, recursive method to find the alphabetically last letter in a string.

I think that I understand the nature of the problem I'm trying to solve, but I don't know how to start with the base case and then the recursion.

Can anyone be willing to solve this problem?

In this case, I would like the following code:

//Method Definition
public static String findZenithLetter(String str) {
   //Put actual working Java code that finds the alphabetically last letter of the desired string here.
   //Use recursion, not loops! :)
   //Don't use arrays! ;)
}

//Driver Code
System.out.println(findZenithLetter("I can reach the apex, at the top of the world."))
//Should print the String "x" if implemented properly

I have tried to attempt numerous, but currently failed ways of solving this problem, including but not limited to:

Sorting the string by alphabetical order then finding the last letter of the new string, excluding punctuation marks.
Using the compareTo() method to compare two letters of the string side by side, but that has yet to work as I am so tempted to use loops, not recursion. I need a recursive method to solve this, though. :)

In the end, the best piece of code that I've written for this problem was just a drawn-out way to compute just the last character of a string and not actually THE alphabetically last character.

CodePudding user response：

This is quite simple. All you need is just iterate (in the recursion of course), and check all characters int he string with local maximum.

public static char findZenithLetter(String str) {
    return findZenithLetter(str, 0, 'a');
}

private static char findZenithLetter(String str, int i, char maxCh) {
    if (i >= str.length())
        return maxCh;

    char ch = Character.toLowerCase(str.charAt(i));

    if (Character.isLetter(ch))
        maxCh = ch > maxCh ? ch : maxCh;

    return findZenithLetter(str, i   1, maxCh);
}

CodePudding user response：

Nibble off the first character at each recursion, returning the greater of it and the greatest found in the rest of the input:

public static String findZenithLetter(String str) {
    if (str.isEmpty()) {
        return ""; // what's returned if no letters found
    }
    String next = str.substring(0, 1);
    String rest = findZenithLetter(str.substring(1));
    return Character.isLetter(next.charAt(0)) && next.compareToIgnoreCase(rest) > 0 ? next : rest;
}

See live demo.

The check for Character.isLetter() prevents non-letter characters, which may be "greater than" letters being returned.

If no letters are found, a blank is returned.