Euler problem number #4

Question

Using Python, I am trying to solve problem #4 of the Project Euler problems. Can someone please tell me what I am doing incorrectly? The problem is to Find the larg

Answer 1

Here is a solution that you might consider. It could be far more efficient but only takes a little time to run.

largest = 0
for a in range(100, 1000):
    for b in range(100, 1000):
        c = a * b
        if str(c) == ''.join(reversed(str(c))):
            largest = max(largest, c)
print(largest)

Answer 2

Here is an efficient general solution (~5x faster than others I have seen):

def pgen(factor):
    ''' Generates stream of palindromes smaller than factor**2 
        starting with largest possible palindrome '''
    pmax = str(factor**2)
    half_palindrome = int(pmax[0:len(pmax)/2]) - 1
    for x in xrange(half_palindrome, 0, -1):
        yield int(str(x) + str(x)[::-1])

def biggest(factor):
    ''' Returns largest palindrome and factors '''
    for palindrome in pgen(factor):
        for f1 in xrange(factor/11*11, factor/10, -11):
            f2 = palindrome/f1
            if f2 > factor:
                break
            if f2*f1 == palindrome:
                return palindrome, f1, f2

>>> biggest(99)
(9009, 99, 91)
>>> biggest(999)
(906609, 993, 913)
>>> biggest(9999)
(99000099, 9999, 9901)
>>> biggest(99999)
(9966006699L, 99979, 99681L)
>>> biggest(9999999)
(99956644665999L, 9998017, 9997647L)
>>> biggest(99999999)
(9999000000009999L, 99999999, 99990001L)
>>> biggest(999999999)
(999900665566009999L, 999920317, 999980347L)