Language: C++

#include <stdio.h>
#include <iostream>
#include <sstream>
#include <algorithm>
#include <functional>
#include <queue>
#include <string.h>
#define FOR(x, z, y) for(int x = (z); y; x++)
#define rep(x, y) for(int x = 0; x < (y); x++)
#define REP(x, z, y) for(int x = (z); x < (y); x++)
#define all(x) x.begin(), x.end()
#define len(x) (sizeof(x) / sizeof(x[0]))
#define split_str(str, sp_word) istringstream stream(str); string res; FOR(cnt, 0, getline(stream,res,sp_word))
#define scanln(x) fgets(x, sizeof x, stdin)
#define down_queue(x) priority_queue<x>
#define up_queue(x) priority_queue<x, vector<x>, greater<x>>
#define i_str(x) atoi(x.c_str())
typedef long long ll;
using namespace std;

int main(){
  int N;
  scanf("%d",&N);
  getchar();

  int a[26],cnt_line[26] = {};
  fill(a,a + 26,51);
  while(N--){
    string line;
    getline(cin,line);
    for(char c:line) {
      a[c - 'a'] = min(a[c - 'a'], count(all(line), c)); 
    }
    for(int i = 'a'; i <= 'z'; i++){
      if(count(all(line), i)){
        cnt_line[i - 'a']++;
      }
    }
  }

  rep(i,26){
    if(cnt_line[i] >= 3 && a[i] != 51){
      rep(j,a[i]){
        printf("%c",i + 'a');
      }
    }
  }

  return 0;
}

Compiler output:
a.cc: In function 'int main()':
a.cc:32:23: error: no matching function for call to 'min(int&, std::__iterator_traits<__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >, void>::difference_type)'
   32 |       a[c - 'a'] = min(a[c - 'a'], count(all(line), c));
      |                    ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
In file included from /usr/include/c++/14/string:51,
                 from /usr/include/c++/14/bits/locale_classes.h:40,
                 from /usr/include/c++/14/bits/ios_base.h:41,
                 from /usr/include/c++/14/ios:44,
                 from /usr/include/c++/14/ostream:40,
                 from /usr/include/c++/14/iostream:41,
                 from a.cc:2:
/usr/include/c++/14/bits/stl_algobase.h:233:5: note: candidate: 'template<class _Tp> constexpr const _Tp& std::min(const _Tp&, const _Tp&)'
  233 |     min(const _Tp& __a, const _Tp& __b)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:233:5: note:   template argument deduction/substitution failed:
a.cc:32:23: note:   deduced conflicting types for parameter 'const _Tp' ('int' and 'std::__iterator_traits<__gnu_cxx::__normal_iterator<char*, std::__cxx11::basic_string<char> >, void>::difference_type' {aka 'long int'})
   32 |       a[c - 'a'] = min(a[c - 'a'], count(all(line), c));
      |                    ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note: candidate: 'template<class _Tp, class _Compare> constexpr const _Tp& std::min(const _Tp&, const _Tp&, _Compare)'
  281 |     min(const _Tp& __a, const _Tp& __b, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algobase.h:281:5: note:   candidate expects 3 arguments, 2 provided
In file included from /usr/include/c++/14/algorithm:61,
                 from a.cc:4:
/usr/include/c++/14/bits/stl_algo.h:5686:5: note: candidate: 'template<class _Tp> constexpr _Tp std::min(initializer_list<_Tp>)'
 5686 |     min(initializer_list<_Tp> __l)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5686:5: note:   candidate expects 1 argument, 2 provided
/usr/include/c++/14/bits/stl_algo.h:5696:5: note: candidate: 'template<class _Tp, class _Compare> constexpr _Tp std::min(initializer_list<_Tp>, _Compare)'
 5696 |     min(initializer_list<_Tp> __l, _Compare __comp)
      |     ^~~
/usr/include/c++/14/bits/stl_algo.h:5696:5: note:   template argument deduction/substitution failed:
a.cc:32:23: note:   mismatched types 'std::initializer_list<_Tp>' and 'int'
   32 |       a[c - 'a'] = min(a[c - 'a'], count(all(line), c));
      |                    ~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~