Sukat

input/code.cpp: In function 'int main()':
input/code.cpp:18:19: error: 'class std::set<std::pair<int, int> >' has no member named 'push_back'
   18 |                 m.push_back({it->S,jt->S});
      |                   ^~~~~~~~~
input/code.cpp:3:11: error: decrement of member 'std::pair<int, int>::first' in read-only object
    3 | #define F first
      |           ^
input/code.cpp:19:21: note: in expansion of macro 'F'
   19 |                 it->F--;
      |                     ^
input/code.cpp:3:11: error: decrement of member 'std::pair<int, int>::first' in read-only object
    3 | #define F first
      |           ^
input/code.cpp:20:21: note: in expansion of macro 'F'
   20 |                 jt->F--;
      |                     ^
input/code.cpp:23:35: error: no matching function for call to 'std::set<std::pair<int, int> >::erase(std::reverse_iterator<std::_Rb_tree_const_iterator<std::pair<int, int> > >&)'
   23 |                         kt=m.erase(kt);
      |...

#include <bits/stdc++.h>
using namespace std;
#define F first
#define S second
string s;
int n,k,c[26];
vector<pair<int,int>> v;
set<pair<int,int>> m;

int main(){
	cin>>s;
	n=s.size();
	for(int e:s) ++c[e-'A'];
	for(int i=0;i<26;++i) m.insert({c[i],i});
	while(m.size()>1){
		auto it=m.rbegin();
		auto jt=it; jt++;
		m.push_back({it->S,jt->S});
		it->F--;
		jt->F--;
		auto kt=m.rbegin();
		while(m.size()>1&&kt->F==0){
			kt=m.erase(kt);
		}
	}

	cout<<v.size()<<'\n';
	for(auto p:v){
		cout<<(char)p.F<<(char)p.S<<'\n';
	}
}