在c++中，出现invalid use of member 'mp' in static member function

Question

我想写一个类实现把字符串按出现次数重新排列，把次数多的放到前面，可是出现：

invalid use of member 'mp' in static member function

class Solution {
public:
    unordered_map<char,int>mp;
    static bool cmp(char a, char b){
        return  mp[a] > mp[b];
    }
    string frequencySort(string s) {
    
        vector<char>num;
        for(auto &ch:s){   //统计每个字符次数，并把出现过的字符放进num
            if(mp.count(ch) == 0)
                num.emplace_back(ch);
            mp[ch]++;
        }
        sort(num.begin(),num.end(),cmp);  //按字符出现次数排序

        vector<string>ret( num.size() );
        int i = 0;
        for(auto &ch: num){
            string s(mp[ch],ch);
            ret[i] += s;
            i++;
        }
        string ans;
        for(auto &str:ret){
            ans += str; 
        }
        return ans;
    }
};

Answer 1

cmp写成static函数的话，就只能访问static变量，但是mp又不能为static变量。所以可以把mp定义到frequencySort函数内，然后排序mp而不是num。如果不考虑出现次数相同的字符的顺序的话，可以这样写：

class Solution {
public:   
    static bool cmp(const pair<char,int> &a, const pair<char,int> &b) {
        return a.second > b.second;
    }
    string frequencySort(string s) {
        unordered_map<char,int> mp; 
      
        vector<char> num;
        for(auto &ch:s){   //统计每个字符次数，并把出现过的字符放进num
            if(mp.count(ch) == 0)
                num.emplace_back(ch);
            mp[ch]++;
        }
        vector<pair<char, int>> elems(mp.begin(), mp.end());
        sort(elems.begin(),elems.end(),cmp);  //按字符出现次数排序
        
        string ans;
        for(auto &p: elems){
            string s(p.second,p.first);
            ans+=s;
        }
        return ans;
    }
};