Language: C++

#include <cstring>
#include <cassert>
#include <utility>
#include <iostream>
#include <cstdio>
#include <iomanip>
#include <bitset>
#include <chrono>
#include <cstdlib>
#include <functional>
#include <tuple>
#include <climits>
#include <limits>
#include <deque>
#include <list>
#include <array>
#include <stack>
#include <queue>
#include <random>
#include <complex>
#include <string>
#include <vector>
#include <set>
#include <map>
#include <unordered_map>
#include <unordered_set>
#include <algorithm>
#include <ext/pb_ds/assoc_container.hpp>
#include <ext/pb_ds/tree_policy.hpp>

using namespace std;
using namespace __gnu_pbds;

#define ll              long long
#define pi              pair<int,int>
#define pll             pair<ll,ll>
#define pb              push_back
#define F               first
#define S               second
#define vi              vector<int>
#define vll             vector<ll>
#define endl            '\n'
#define all(v)          (v).begin(),(v).end()
#define FOR(i,a,b)      for(int i=a;i<b;i++)
#define MP(a,b)         make_pair(a,b)
#define debug(x)        cerr<< #x << " : " << x << endl;
#define debugp(p)       cerr<< #p << " : " << "(" << (p).first << "," << (p).second << ")\n";
#define debugv(v)       cerr<< #v << " : " << "["; for(int i = 0; i < (v).size(); i++) cerr << v[i] << " "; cerr<<"]\n";
#define debugs(m)       cerr<< #m << " : [ "; for(auto itr = m.begin(); itr != m.end(); itr++) cerr <<*itr << " "; cerr << "]\n";
#define debugm(m)       cerr<< #m << " : [ "; for(auto itr = m.begin(); itr != m.end(); itr++) cerr <<"(" << itr->first << "," << itr->second << ") "; cerr << "]\n";

/* ordered set: 
tree<T, null_type,less<T>, rb_tree_tag,tree_order_statistics_node_update>
member functions :
1. order_of_key(k) : number of elements strictly lesser than k
2. find_by_order(k) : k-th element in the set
*/

inline ll modexp(ll x, ll ex , ll md) {
    ll ans = 1ll;
    while (ex > 0) {
        if (ex & 1ll) ans = (ans * x) % md;
        ex >>= 1ll;
        x = (x * x) % md;
    }
    return ans;
}

const int inf = 1e9 + 7;

void solve(){
    int h , w , m;
    cin >> h >> w >> m;
    map<int, multiset<int>> xs;
    map<int,int> ycnt;
    for (int i = 0; i < m; i++){
        int x, y;
        cin >> x >> y;
        xs[x].insert(y);
        ycnt[y]++;
    }

    int ans = 0;
    for (auto x : xs){
        set<int> st = x.second;
        for (auto y : st){
            --ycnt[y];
            if (ycnt[y] == 0){
                ycnt.erase (ycnt.find (y));
            }
        }
        int mxy = 0;
        if (int(ycnt.size()) != 0){
            auto it = ycnt.end(); it--;
            mxy = it->first;
        }
        if (mxy != 0)
            ans = max (ans , int(st.size()) + ycnt[mxy]);
        else ans = max (ans , int (st.size()));
        for (auto y : st){
            ++ycnt[y];
        }
    }
    cout << ans << endl;
}
int main(){
    ios :: sync_with_stdio(false); 
    cin.tie(nullptr); 
    cout.tie(nullptr);
    
    int T = 1;
    // cin >> T;
    for(int tc = 1; tc <= T; tc++){
        // cout << "Case #" << tc << ": ";
        solve();
    }
    #ifdef LOCAL
    cerr << "Time elapsed\t: " << 1000 * ((double)clock()) / (double)CLOCKS_PER_SEC << "ms\n";       
    #endif
}



Compiler output:
a.cc: In function 'void solve()':
a.cc:85:25: error: conversion from 'std::multiset<int>' to non-scalar type 'std::set<int>' requested
   85 |         set<int> st = x.second;
      |                       ~~^~~~~~