位运算：单词长度的最大乘积

第一版：

class Solution {
public:
    int maxProduct(vector<string>& words) {
        int num = words.size();
        int ans = 0;
        for(int i = 0; i < num; i++){
            int m = words[i].size(); 
            for(int j = i + 1; j < num; j++){
                int n = words[j].size();
                if(!has_same_word(words[i], words[j])){
                    ans = max(ans, m * n);  
                }
            }
        }
        return ans;
    }

    bool has_same_word(string str1, string str2){
        vector<int> alpha(26, 0);
        for(int i = 0; i < str1.size(); i++){
            alpha[str1[i] - 'a'] = 1;
        }
        for(int j = 0; j < str2.size(); j++){
            if(alpha[str2[j] - 'a'] == 1){
                return true;
            }
        }
        return false;
    }
};

第二版：

class Solution {
public:
    int maxProduct(vector<string>& words) {
        int len = words.size();
        vector<int> save(len , 0);
        for(int i = 0; i < len; i++){
            int maskbit = 0;
            for(char c : words[i]){
                maskbit |= (1 << (c - 'a'));
            }
            save[i] = maskbit;
          //  cout<<save[i]<<endl;
        }

        int ans = 0;
        for(int i = 0; i < len; i++){
            int m = words[i].size(); 
            for(int j = i + 1; j < len; j++){
                if(!(save[i] & save[j])){
                    int n = words[j].size();
                    ans = max(ans, m * n);  
                   // cout<<"m =  "<<m<<"   , n =  "<<n<<endl;
                }
            }
        }
        return ans;
    }

    bool has_same_word(string str1, string str2){
        int ans1 = 0, ans2 = 0;
        for(int i = 0; i < str1.size(); i++){
            ans1 |= 1<<(str1[i] - 'a');
        }
        for(int j = 0; j < str2.size(); j++){
            ans2 |= 1<<(str2[j] - 'a');
        }
        if(ans1  & ans2)return true;
        else return false;
    }
};

第三版：

class Solution {
public:
  
  int maxProduct(vector<string>& words) {
        int len = words.size();
        unordered_map<int, int> mask_map;
        for(int i = 0; i < len; i++){
            int maskbit = 0;
            for(char c : words[i]){
                maskbit |= (1 << (c - 'a'));
            }
            if(mask_map.count(maskbit)){
                int max_val = max(mask_map[maskbit], (int)words[i].size()); //把第二个参数转成和第一个参数一样的 int 型，问题得以解决
                mask_map[maskbit] = max_val;
            }
            else{
                mask_map[maskbit] = words[i].size();
            }
        }

        int ans = 0;
        for(auto x : mask_map){
            for(auto y : mask_map){
                if((x.first & y.first) == 0)
                ans = max(ans, x.second * y.second);  
            }
        }
        return ans;
    }

    bool has_same_word(string str1, string str2){
        int ans1 = 0, ans2 = 0;
        for(int i = 0; i < str1.size(); i++){
            ans1 |= 1<<(str1[i] - 'a');
        }
        for(int j = 0; j < str2.size(); j++){
            ans2 |= 1<<(str2[j] - 'a');
        }
        if(ans1  & ans2)return true;
        else return false;
    }
};