map and set are two common associative containers. By default, common data types
can be directly used as the key and their comparing method is std::less. On occassions,
we need to store user-defined type as the key. We need to define the comparing method for
the type.
Main course
There are two ways that we can set the comparing functions.
- A struct class which defines
operator() - A lambda function
#include <iostream>
#include <vector>
#include <map>
#include <string>
#include <functional>
using namespace std;
class Solution {
struct Cmp {
bool operator()(const string& l, const string& r) const {
string ll = l;
string rr = r;
sort(begin(ll), end(ll));
sort(begin(rr), end(rr));
cout << "ll: " << ll <<endl;
cout << "rr: " << rr << endl;
return ll.compare(rr) < 0;
}
};
public:
vector<vector<string>> groupAnagrams(vector<string>& strs) {
const function<bool(const string&, const string&)> cmp = [](const string& l, const string& r) {
string ll = l;
string rr = r;
sort(begin(ll), end(ll));
sort(begin(rr), end(rr));
cout << "ll: " << ll <<endl;
cout << "rr: " << rr << endl;
return ll.compare(rr) < 0;
};
/* Method 1: */
map<string, vector<string>, decltype(cmp)> m(cmp);
/* Method 2: */
map<string, vector<string>, Cmp> m1;
vector<vector<string>> result;
for (string& s: strs) {
m[s].push_back(s);
}
for (auto& x : m) {
result.push_back(x.second);
}
return result;
}
};
int main() {
vector<string> s{"hi", "ih"};
Solution sol;
sol.groupAnagrams(s);
return 0;
}
- If we choose to use a struct type,
operator()must be decorated byconst(not the argument)- Otherwise you will get a compiliation error
- If we we choose to use a lambda function, we have to pass the function to the constructor
- Otherwise you will get a wierd error:
bad_function_call
- Otherwise you will get a wierd error: