谁能告诉我这段代码是如何工作的？

Question

I can't figure out how this recursion problem works.

I can only get to the point where it prints "teal" after that I get lost.

class Anagrams {

    private static void printAnagramsHelper(String prefix, String word) {

        if (word.length() <= 1) {
            System.out.println(prefix + word);
        } else {

            printAnagramsHelper(
                                prefix + word.charAt(0),
                                word.substring(1)
                                );

            for (int i = 1; i <= word.length() - 2; i++) {
                printAnagramsHelper(
                                    prefix + word.charAt(i),
                                    word.substring(0, i) + word.substring(i + 1)
                                   );
            }

            printAnagramsHelper(
                                prefix + word.charAt(word.length() - 1),
                                word.substring(0, word.length() - 1)
                                );
        }
    }

    public static void printAnagrams(String word) {
        printAnagramsHelper("", word);
    }

    public static void main(String[] args) {
        printAnagrams("teal");
    }
}

Output is correct but cant figure out how did it come.

Answer 1

The “prefix” is the result built so far, and “word” is the pool of letters to add the prefix.

The terminating case is when there are no more letters in “word”, so the result (“prefix”) is printed and recursion stops.

There are three ways of recursing:

1. Add the first letter of “word” to “prefix” and recurse with the rest of “word”
2. Iterate from the 2nd letter of “word” to the 2nd last letter, recurse with that letter “removed” (by adding substrings either side of the removed letter together)
3. Add the last letter of “word” to “prefix” and recurse with the preceding letters of “word”

The code is the java implementation of the above description.