Given a string array words, find the maximum value of length(word[i]) * length(word[j]) where the two words do not share common letters. You may assume that each word will contain only lower case letters. If no such two words exist, return 0.

Example 1:

Given ["abcw", "baz", "foo", "bar", "xtfn", "abcdef"]

Example 2:

Given ["a", "ab", "abc", "d", "cd", "bcd", "abcd"]
Return 4
The two words can be "ab", "cd".

Example 3:

Given ["a", "aa", "aaa", "aaaa"]
Return 0
No such pair of words.

Credits:
Special thanks to @dietpepsi for adding this problem and creating all test cases.

解題思路：本題主要是如何快速求出兩個字符串有沒有交集。用一個int值標記每個串a-z哪些字母出現過，之後兩兩比較看是否有交集

class Solution {
public:
    int maxProduct(vector<string>& words) {
        vector<int>v(words.size()+1,0);
        for(int i=0;i<words.size();i++){
            for(int j=0;j<words[i].length();j++){
                v[i]
 
|=1<<words[i][j];
            }
        }
        int mmax=0;
        for(int i=0;i<words.size();i++){
            for(int j=i+1;j<words.size();j++){
                if(!(v[i]&v[j])&&words[i].length()*words[j].length()>mmax){
                    mmax=words[i].length()*words[j].length();
                }
            }
        }
        return mmax;
    }
};

318. Maximum Product of Word Lengths