Largest Palindrome Product

Problem 4

Problem definition

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

\[\begin{align} 9009 = 91 \times 99 \end{align}\]

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

Solution

function isPalindrome(num::Int64)
    x = Vector{Int}(undef, 18)
    for i in eachindex(x)
        x[i] = num % 10
        num ÷= 10
    end
    y = x[1:findlast(i -> i > 0, x)]
    for i in eachindex(y)
        if !(y[i] == y[end - i + 1])
            return false
        end
    end
    return true
end

function main()
    current = 0
    idx = (0, 0)
    for i in 999:-1:900
        for j in 999:-1:900
            new = i*j
            if (new > current && isPalindrome(new))
                current = new
                idx = (i, j)
            end
        end
    end
    println("$(current) = $(idx[1]) x $(idx[2])")
end
main()

906609 = 993 x 913