STL sort function in C++ wrt strings - c++

Question

So I've been trying to sort a string based on the frequency of its characters. However the online judge I've been using shows me the error
Line 17: invalid use of non-static member function 'bool olution::helper(char, char)'
Why is the call to my function wrong? I have used the sort() function before, but not to strings. Is my helper() function incorrect?
class Solution {
public:
unordered_map<char,int> freq;
bool helper(char c1,char c2){
if(freq[c1]>freq[c2]) return false;
else return true;
}
string frequencySort(string s) {
for(char c:s)
{
freq[c]++;
}
sort(s.begin(),s.end(),helper);
return s;
}
};

Answer 1

Use a lambda to capture this:
sort(s.begin(),s.end(),[this](auto a, auto b) -> bool { return helper(a,b); });

Answer 2

Why is the call to my function wrong? I have used the sort() function
before, but not to strings. Is my 'helper()' function incorrect?
Because helper is member function of Solution. When you do this
sort(s.begin(),s.end(),helper);
you are basically doing this
sort(s.begin(),s.end(),this->helper);
The 3rd parameter to sort needs to be a standalone function, a predicate, a functor or a lambda. It cannnot be a non-static member of a class
This code, cleaned up, works. Note the statics
class Solution {
public:
// using thread_local so that each thread
// has its own global variable.
static thread_local std::unordered_map<char, int> freq;
static bool helper(char c1, char c2) {
return (freq[c1]<freq[c2]);
}
std::string frequencySort(std::string s)
{
freq.clear();
for (char c : s)
++freq[c];
std::sort(s.begin(), s.end(), helper);
return s;
}
};
// definition
std::unordered_map<char, int> Solution::freq;

Answer 3

Member functions have a hidden parameter that becomes this. You need either expose the state more widely, or write a capturing lambda
Also a Compare predicate must return false if you are comparing a value to itself, yours does not.
class Solution {
public:
string frequencySort(string s) {
unordered_map<char,int> freq;
for(char c:s)
{
freq[c]++;
}
sort(s.begin(),s.end(),[&freq](char lhs, char rhs){ return freq[lhs] < freq[rhs]; });
return s;
}
};