Maximize Score After N Operations — Python

# Time:  O(n^2 * 2^n)
# Space: O(2^n)

import itertools
from fractions import gcd


class Solution(object):
    def maxScore(self, nums):
        """
        :type nums: List[int]
        :rtype: int
        """
        def popcount(n):
            count = 0
            while n:
                n &= n-1
                count += 1
            return count

        def bits(mask):
            result = []
            i = 0
            while mask:
                if mask&1:
                    result.append(i)
                i += 1
                mask >>= 1
            return result
            
        dp = [0]*(2**len(nums))
        for mask in xrange(3, len(dp)):
            cnt = popcount(mask)
            if cnt%2:
                continue
            for i, j in itertools.combinations(bits(mask), 2):  # Time: O(n^2)
                dp[mask] = max(dp[mask], cnt//2*gcd(nums[i], nums[j]) + dp[mask^(1<<i)^(1<<j)])
        return dp[-1]