Sukat

input/code.cpp: In function 'int main()':
input/code.cpp:3:36: warning: comparison of integer expressions of different signedness: 'int' and 'std::__cxx11::basic_string<char>::size_type' {aka 'long unsigned int'} [-Wsign-compare]
    3 | #define forn(i, n) for(int i = 0; i<n; ++i)
      |                                    ^
input/code.cpp:38:5: note: in expansion of macro 'forn'
   38 |     forn(i, r.size()) {
      |     ^~~~

#include <bits/stdc++.h>

#define forn(i, n) for(int i = 0; i<n; ++i)
#define FOR(i, k, n) for(int i = k; i<n; ++i)
#define F first
#define S second
#define pb push_back

using namespace std;

int n,a[30];
string s,r;

int main() {

    cin >> s;
    n = s.size();

    for (char i : s) {
        a[i-'A']++;
    }

    forn(gtg, (int)(n/2)) {
        int besti = 29, bestj = 29;
        forn(i, 30) {
            if (a[besti] < a[i]) besti = i;
        }
        forn(i, 30) {
            if (a[bestj] < a[i] && besti != i) bestj = i;
        }
        if (!(a[besti]) || !(a[bestj])) break;

        r.pb((char)(besti+'A'));
        r.pb((char)(bestj+'A'));
        a[besti]--; a[bestj]--;
    }
    cout << r.size()/2 << endl;
    forn(i, r.size()) {
        cout << r[i];
        if (i&1) cout << endl;
    }

    return 0;
}

Test 1

Group: 1, 2, 3

Verdict: ACCEPTED

Test 2

Group: 1, 2, 3

Verdict: ACCEPTED

Test 3

Group: 1, 2, 3

Verdict: ACCEPTED

Test 4

Group: 1, 2, 3

Verdict: ACCEPTED

Test 5

Group: 1, 2, 3

Verdict: ACCEPTED

Test 6

Group: 1, 2, 3

Verdict: ACCEPTED

Test 7

Group: 1, 2, 3

Verdict: ACCEPTED

Test 8

Group: 2, 3

Verdict: ACCEPTED

Test 9

Group: 2, 3

Verdict: ACCEPTED

Test 10

Group: 2, 3

Verdict: ACCEPTED

Test 11

Group: 2, 3

Verdict: ACCEPTED

Test 12

Group: 2, 3

Verdict: ACCEPTED

Test 13

Group: 2, 3

Verdict: ACCEPTED

Test 14

Group: 2, 3

Verdict: ACCEPTED

Test 15

Group: 3

Verdict: ACCEPTED

Test 16

Group: 3

Verdict: ACCEPTED

Test 17

Group: 3

Verdict: ACCEPTED

Test 18

Group: 3

Verdict: ACCEPTED

Test 19

Group: 3

Verdict: ACCEPTED

Test 20

Group: 3

Verdict: ACCEPTED

Test 21

Group: 3

Verdict: ACCEPTED