找出由两个 2 位数字的乘积构成的最大回文

Question

def largest_palindrome(n1,n2):
    mylist = [x for x in range(n1,n2)]
    for y in mylist:
        if y == y[::-1]:
            print(y)
        else:
            pass    

largest_palindrome(100,9801)

When I am executing this code the error that comes up is TypeError: 'int' object is not subscriptable . I need to know what is the problem is in this code and what changes will be done to make this code running.

Answer 1

You need to cast to string to be able to reverse and compare:

def largest_palindrome():                        # <-- arguments are not needed
    for y in (x for x in range(9801, 100, -1)):  # use a generator, that iterates from largest to smallest. 
        if str(y) == str(y)[::-1]:
            return y                             # early exit when largest match is found (it will be the first)

print(largest_palindrome())

spoiler alert:

9779

as a one liner:

max([x for x in range(9801, 100, -1) if str(x) == str(x)[::-1]])

As a one liner generator

(thanks @Austin in the comments):

next(x for x in range(9801, 100, -1) if str(x) == str(x)[::-1])

Answer 2

Reblochon's answer doesn't solve the problem as it only iterates between the smallest and the biggest number that could come from two two-digit numbers. It doesn,t iterate through two-digit numbers.

def largest_palindrome():
    lastBiggestProduct = 0;
    lastBiggestNumb = 10;
    for firstNum in range(10,100):
        a = list(range(lastBiggestNumb,firstNum))
        a.extend(range(firstNum+1,100))
        for secondNum in a:
            prod = firstNum*secondNum
            if(prod>lastBiggestProduct and str(prod) == str(prod)[::-1]):
                lastBiggestProduct = firstNum*secondNum
                lastBiggestNumb = secondNum
    return lastBiggestProduct


print(largest_palindrome())

That returns:

9009