410 遞迴字串替換

import java.util.Scanner;
public class JPA410 {
    static Scanner keyboard = new Scanner(System.in);
    public static void main(String args[]) {
        String s, c1, c2;
        System.out.print("Input a string: ");
        s = keyboard.nextLine();
        System.out.print("Input a character: ");
        c1 = keyboard.nextLine();
        System.out.print("Input another character: ");
        c2 = keyboard.nextLine();
        System.out.printf("%s\n", replace(s, c1, c2));
    }

    static String replace(String s,String c1,String c2){
    if(s.equals("")){
    return "";
    }
    else if (s.substring(0,1).equals(c1)){
    return c2+replace(s.substring(1),c1,c2);
    }
    else{
    return s.substring(0,1)+replace(s.substring(1),c1,c2);
    }
    }
}