Solution to Project Euler 4


Here’s the description for Problem 4 on Project Euler:

A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 99.

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

As usual the brute force approach was my first guess, and it worked pretty well. I just implemented a small function to test whether the number is palindrome (by inverting it and comparing the result with itself), and then I tested all the multiples of number with 3 digits. Code is below:


#include <stdio.h>
#include <math.h>

int isPali(n){
    int copy = n;
  int complement = 0;

  while (copy>0){
        complement = complement * 10 + copy % 10;
        copy/=10;
  }

  if (complement==n)
        return 1;
    return 0;
}

int main(){
    int i,j,ans,x;
    ans=0;

    for (i=1;i<1000;i++){
        for (j=1;j<1000;j++){
            x=i*j;
            if (isPali(x)&&x>ans)
                ans=x;
            }
        }

    printf("%dn",ans);
return 0;
}


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