Project Euler 4: Find the largest palindrome product

Project Euler 4 Description

Project Euler 4: 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.

Analysis

We are tasked with finding the largest palindromic number with two 3 digit factors. One method is searching factors downwards from 999 as it is more likely to find a larger product from larger factors. We test each product as a palindrome and save it and continue until we exhaust all our potential factors.

Nanogyth’s comment below demonstrates how to speed things up significantly. It summarizes that if a six digit number is palindromic it must follow the form:

Where a, b, and c are integers such that 0 < a ≤ 9, 0 ≤ b, c ≤ 9

This means that one of our target factors has to be divisible by 11 and allows us to step our search index, j, by 11 when i is divisible by 11. When i is not divisible by 11 we have to decrement j by 1.

Here’s my initial simple brute-force solution; about 0.3 seconds, but not fast enough for bigger problems.

print max(i*j for i in xrange(1000) for j in xrange(i,1000) if str(i*j)==str(i*j)[::-1])

The final solution is presented below and simply performs an exhaustive investigation of 3-digit factors starting from the biggest factors until a maximum palindrome product is found. There’s an early out condition that breaks the inside loop (j) by finding a palindrome less than the current maximum. This is allowed because palindrome products are found in descending order from the perspective of the inside loop.

One subtle optimization was to check the limit before checking if the product is a palindrome as the palindrome check is more computationally expensive. If the first condition is false the succeeding conditions are ignored.

if p < L and is_palindromic(p): x, y, pmax = i, j, p

The two factors are printed along with their product. L was added to solve the HackerRank Project Euler 4 version which keeps the search to 3 digit factors but includes a limit to the palindrome product.

This program and method
solves all test cases for
Project Euler 4 on HackerRank

Project Euler 4 Solution

Afterthoughts

• Function is_palindromic is listed in Common Functions and Routines for Project Euler
  
• There are only 279 discrete 6-digit palindromes which are products of two 3-digit numbers.
  
• The largest palindrome for some other cases (Note: I restricted the search range to something sane):
  

You can see that this method is becoming impractical for a larger number of digits.