Problem 235. Project Euler: Problem 4, Palindromic numbers

Created by Doug Hull

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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 numbers less than or equal to the input number.
Thank you to Project Euler Problem 4

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Last Solution submitted on Jan 05, 2026

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Alan Chalker on 16 Feb 2012

Your last test case is accidentally all commented out, so isn't a real test case

Jan-Andrea Bard on 1 Nov 2021

The question is unclear. Do the factor-numbers all have to be two-digited?

Dyuman Joshi on 2 Nov 2021

@Jan-Andrea Bard, no.
Although it is not mentioned clearly in the problem statement but you can see a hint from the test suite

Brandon on 22 May 2023

This problem would be really fun with larger test cases. It would eliminate kron() and stop naively checking every string combination.

Christian Schröder on 24 Dec 2023

@Brandon you could submit a new problem ("Palindromic Numbers - Hard Mode" or so).

Yu Zhang on 18 Dec 2025

Hello,

Just want to make a correction and make a clear problem statement. It is missing a multiplication sign.

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

Modified to be,

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

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Problem 235. Project Euler: Problem 4, Palindromic numbers

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Problem 235. Project Euler: Problem 4, Palindromic numbers

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