Question

I am trying to solve the following problem:

Find the largest palindrome made from the product of two 3-digit numbers.

I have the following Java code:

public static void main(String[] args) {
        int a = 999, b = 999;
        for(int i = 100; i <= a; i++) {
            for(int j = 100; j <= b; j++) {
                checkPalindrome(i*j, i, j);
            }
        }
    }

    public static void checkPalindrome(int n, int a, int b) {
        String s = "" + n;
        boolean palindrome = false;
        int j = s.length()-1;

        for(int i = 0; i < s.length(); i++){
            if(s.charAt(i) != s.charAt(j))
                break;
            j -= i;
        }

        if(palindrome)
            System.out.println(n + ", " + a + ", " + b);
    }

I’m still lacking the change of the “palindrome” variable but at the moment if I run it I get a String index out of range on line 28 which is the j -= i I just don’t understand why this is happening I mean, I get that the difference is resulting in a number lower than 0 but I can’t figure out WHY it happens. Could someone please explain me?

Answer 1

Your method can be improved like this. The condition in for loop i<=j reduced number of iterations too.

public static void checkPalindrome(int n, int a, int b) {
    String s = "" + n;
    boolean palindrome = false;
    int j = s.length()-1;

    for(int i = 0; i <= j; i++){
        if(s.charAt(i) != s.charAt(j))
            break;
        j --;
    }

    if(palindrome)
        System.out.println(n + ", " + a + ", " + b);
}

Hope this helps.