Carebear Carebear - 2 months ago 8
Java Question

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

I am having a bit of trouble solving Project Euler question 4. I am inexperienced in programming and didn't really understand other answers. I managed to write a code that prints all six-digits palindromes. How do I find the largest palindrome made from the multiplying of two 3-digit numbers?

public class Main {

public static void main(String[] args) {
int num = 998001;
int count=0;
int temp1, temp2, temp3, temp4, temp5, temp6;
int temp;
int num1=999;
int num2=100;
for (int i = 100000; i <= 998001; i++) {
num=998001-count;
temp=num;
temp1=temp%10;
temp=temp/10;
temp2=temp%10;
temp=temp/10;
temp3=temp%10;
temp=temp/10;
temp4=temp%10;
temp=temp/10;
temp5=temp%10;
temp=temp/10;
temp6=temp%10;
temp=temp/10;

if (temp1==temp6 && temp5==temp2 && temp3==temp4) {
System.out.println(num);
}
count=count+1;
}
}
}

Answer

Here is a method which loops over all numbers from 1-999 and multiplies them with all possible numbers from 1-999. You will need to define a method isPalindrome(int) that will check if a given int is a palindrome.

//save the largest number
int largest = 0;

//loop over every possible product of numbers from 100-999
for (int i = 999; i >= 100; i--) {
 for (int j = i; j >= 100; j--) {
  int curr = i * j;

  //if the current number is a palindrome and is greater than the last found largest
  if (isPalindrome(curr) && curr > largest) {

   //save it as the new largest found number
   largest = curr;
  }
 }
}

The isPalindrome method might look like this:

private static boolean isPalindrome(Integer possible) {
    String toStr = possible.toString();
    int len = toStr.length();
    if (len % 2 == 1) {
        len = len - 1;
    }

    for (int i = 0; i < len / 2; i++) {
        if (toStr.charAt(i) != toStr.charAt(toStr.length() - (1 + i))) {
            return false;
        }
    }

    return true;
}