# Exercise: 4 palindromic numbers

Exercise: 4 palindromic numbers /* A palindromic number reads the same both ways. The largest palindrome made from the product of two 2-digit numbers is 9009 = 91 x 99. Find the largest palindrome made from the product of two 3-digit numbers. */ // The only way to work this out is to try all pairs of 3 digit numbers (100 to // 999). However, we can trim the search space by testing a number with all // numbers higher than it (e.g. test 998 with 998-999, 997 with 997-999), so we // are testing all unique combinations), so we // are testing all unique combinations palindromes = [] (100..999).each { i -> (i..999).each { j -> product = i * j // NOTE: convert string just with interpolation, could also do it with // ``(String)`` prod_str = "\${product}" // Grab the first & second halves of string, reverse second & compare len = prod_str.size() half = (int) (len / 2) // NOTE: ``substring`` is a little like a python slice - where do you str_a = prod_str.substring (0, half) // start, where do you stop just short of str_b = prod_str.substring (len - half, len).reverse() if (str_a == str_b) { palindromes << product } } } println palindromes.max()