[LeetCode Python3] 318. Maximum Product of Word Lengths + 位掩码

318. Maximum Product of Word Lengths

from collections import defaultdict

class Solution:
    def maxProduct(self, words: List[str]) -> int:
        hashmap = defaultdict(int)
        bit_number = lambda ch: ord(ch) - ord('a')

        for word in words:
            bitmask = 0
            for ch in word:
                # add bit number bit_number in bitmask
                bitmask |= 1 << bit_number(ch)
            # there could be different words with the same bitmask
            # ex. ab and aabb
            hashmap[bitmask] = max(hashmap[bitmask], len(word))

        max_prod = 0
        for x in hashmap:
            for y in hashmap:
                if x & y == 0:
                    max_prod = max(max_prod, hashmap[x] * hashmap[y])
        return max_prod

方法小结：
1、位掩码：
用整数的低26位来代替单词中是否包含相应的字母；当两个单词中包含相同字母时，他们的掩码相与不为0。
2、预运算
用预运算来避免重复计算。