map<string,int> hash;
//等价于 map<string,int, less<string>> hash;

map<string,int, greater<string> > hash;

bool cmp(pair<string,int> a, pair<string, int> b) {
    return a.second < b.second;//从小到大排序
}

map<string,int> m;
m["a"] = 2;
m["b"] = 3;
m["c"] = 1;
vector<pair<string,int> > vec(m.begin(),m.end());
/*
 *vec(m.begin(),m.end()代表用map的所有元素创建vector
*/
sort(vec.begin(),vec.end(),cmp);