如何使用递归来反转字符串？

Question

    public String reverse(String word) {

    if ((word == null) || (word.length() <= 1)) {
        return word;
    }
    return reverse(word.substring(1)) + word.charAt(0);
}

I have this code that professor sent me but I don't get it. I know what recursion is but I'm still a newbie at Java Programming so if anybody would care to explain to me the part return reverse(word.substring(1)) + word.charAt(0);

what does the subString(1) does and the chartAt(0)?

Answer 1

The way the recursive part of this works is that to reverse a string, you remove the first character, reverse what's left, and then append the first character to the result. That's what the prof's code is doing.

• word.substring(1) returns the substring starting at index 1 and going to the end
• word.charAt(0) returns the character at index 0

There's a bit more going on when the two pieces are appended using + . The issue is that word.charAt(0) has a return type of char . Since the left-hand part of the + is a String , the Java language rules say that the right-hand side must be converted to a String if it isn't one. So the char value is first converted to a Character and then the toString() method of the Character class is called. This returns a String consisting of the single character.

It might have been more efficient code to write that line like:

return reverse(word.substring(1)) + word.substring(0, 1);

The two-argument version of substring returns the substring between the two indexes. That would eliminate the autoboxing and conversion to String .

Answer 2

This is recursion . Here are documentation for subString() and charAt() . Coming to how this works:

public static String reverse(String word) {

    if ((word == null) || (word.length() <= 1)) {
        return word;
    }
    return reverse(word.substring(1)) + word.charAt(0);
}

Pass1: reverse("user") : return reverse("ser")+'u';

Pass2: reverse("ser")+'u' : return reverse("er")+'s'+'u';

Pass3: reverse("er")+'s'+'u' : return reverse("r")+'e'+'s'+'u';

Pass4: reverse("r")+'e'+'s'+'u' : return 'r'+'e'+'s'+'u'; // because here "r".length()==1

Answer 3

return reverse(word.substring(1)) + word.charAt(0);

you should read it this way:

• remove the first letter away from the word
• reverse the rest (recursive call)
• put the first letter at the end

if you assume this function reverses the strings of length N, you can easily see that it must reverse the strings of length N+1. If you realize that the word with at most one letter is the same if reversed (the first three lines of code), you have a complete very simple proof using Mathematical Induction that this function really reverses the string.