Language: C++

 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
 #include <iostream>
#include <cstdio>
#include <algorithm>
#include <cmath>
#include <vector>
#include <list>
#include <set>
#include <map>
#include <queue>
#include <stack>
#include <cctype>
#include <cassert>
#include <climits>
#include <string>
#include <bitset>
#include <cfloat>
#include <unordered_set>
#include <iomanip>
using namespace std;
typedef long double ld;
typedef long long int ll;
typedef unsigned long long int ull;
typedef pair<int,int> pii;
typedef vector<int> vi;
typedef vector<char> vc;
typedef vector<bool> vb;
typedef vector<double> vd;
typedef vector<string> vs;
typedef vector<ll> vll;
typedef vector<pair<int,int> > vpii;
typedef vector<vector<int> > vvi;
typedef vector<vector<char> > vvc;
typedef vector<vector<string> > vvs;
typedef vector<vector<ll> > vvll;
#define rep(i,n) for(int i = 0; i < (n); ++i)
#define rrep(i,n) for(int i = 1; i <= (n); ++i)
#define irep(it, stl) for(auto it = stl.begin(); it != stl.end(); it++)
#define drep(i,n) for(int i = (n) - 1; i >= 0; --i)
#define fin(ans) cout << (ans) << endl
#define mp(p,q) make_pair(p, q)
#define pb(n) push_back(n)
#define all(a) a.begin(), a.end()
#define rall(a) a.rbegin(), a.rend()
#define Sort(a) sort(a.begin(), a.end())
#define Rort(a) sort(a.rbegin(), a.rend())
#define MATHPI acos(-1)
int dx8[8] = {1, 0, -1, 0, 1, -1, -1, 1};
int dy8[8] = {0, 1, 0, -1, 1, 1, -1, -1};
int dx4[4] = {1, 0, -1, 0};
int dy4[4] = {0, 1, 0, -1};
template <class T> inline bool chmax(T& a,T b){if(a<b){a=b;return 1;} return 0;}
template <class T> inline bool chmin(T& a,T b){if(a>b){a=b;return 1;} return 0;}
template<typename A, size_t N, typename T> void Fill(A (&array)[N], const T &val){ fill((T*)array, (T*)(array+N), val); }
struct io{io(){ios::sync_with_stdio(false);cin.tie(0);}};
const int INF = INT_MAX;
const ll LLINF = 1LL<<60;
const ll MOD = 1000000007;
const double EPS = 1e-9;


int H,W;
vvi S;


vi dist;
queue<int> que;

int bfs(int i,int j){

   vvi dist(H,vi(W,-1));//distとqueueは毎回ここで初期化した方がtempを作るよりわかりやすい
   queue<pii> que;

   int distMax = 0;
   dist[i][j]=0;
   que.push({i,j});
   while(!que.empty()){
       pii v = que.front();
       que.pop();
       int curX = v.first,curY =v.second;
       distMax = max(distMax,dist[curX][curY]);
       
       for(auto dx:dx4){
           for(auto dy:dy4){
               int nX = curX + dx,nY = curY + dy;
               if(nX < 0 || nX >= W || nY < 0 || nY >= H) continue;
               if(S[nx][ny] == '#') continue;

               if(dist[nX][nY] != -1){
                   dist[nX][nY] = dist[curX][curY]+1;
               }
               

               que.push({nX,nY});
           }
       }
   }

   return distMax;
   
}



int main(){
   cin >> H >> W;
   S.assign(H,vi(W));
  
   //ただGにいけるマスに達したら必ずGに行かなくてはいけない
   rep(i,H){
       rep(j,W){
           cin >> S[i][j];
       }
   }
   
   int countMax=0;
   rep(i,H){
       rep(j,W){
           countMax = max(countMax,bfs(i,j));
       }
   }
   fin(countMax);
    
}
  

Compiler output:
a.cc: In function 'int bfs(int, int)':
a.cc:102:21: error: 'nx' was not declared in this scope; did you mean 'nX'?
  102 |                if(S[nx][ny] == '#') continue;
      |                     ^~
      |                     nX
a.cc:102:25: error: 'ny' was not declared in this scope; did you mean 'nY'?
  102 |                if(S[nx][ny] == '#') continue;
      |                         ^~
      |                         nY