Reverse a string in Java, in O(1)?

Question

Is there any facility in the standard Java libraries that, given a CharSequence, produces the reverse in O(1) time?

I guess this is \"easy\" to implement, just wond

Answer 1

Well, you can easily produce an implementation of CharSequence which returns the same length, and when asked for a particular character returns the one at length-index-1. toString() becomes O(n) of course...

Creating that reversed CharSequence would be O(1) - all it's got to do is store a reference to the original CharSequence, after all. Iterating over all the characters within the sequence is going to be O(n) though, obviously.

Note that creating a reversed CharSequence (as per the body of your question) is not the same as creating a reversed String (as per the title of your question). Actually producing the String is O(n), and has to be.

Sample code, mostly untested:

public final class ReverseCharSequence implements CharSequence
{
    private final CharSequence original;

    public ReverseCharSequence(CharSequence original)
    {
        this.original = original;
    }

    public int length()
    {
        return original.length();
    }

    public char charAt(int index)
    {
        return original.charAt(original.length() - index - 1);
    }

    public CharSequence subSequence(int start, int end)
    {
        int originalEnd = original.length() - start;
        int originalStart = original.length() - end;
        return new ReverseCharSequence(
            original.subSequence(originalStart, originalEnd));
    }

    public String toString()
    {
        return new StringBuilder(this).toString();
    }
}