Language: C++

#pragma GCC optimize("O3")
#pragma comment(linker, "/stack:200000000")
#pragma GCC optimize("unroll-loops")
 
#include <algorithm>
#include <bitset>
#include <cassert>
#include <chrono>
#include <cmath>
#include <complex>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <deque>
#include <functional>
#include <iomanip>
#include <iostream>
#include <iterator>
#include <limits>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <random>
#include <ratio>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <unordered_map>
#include <unordered_set>
#include <utility>
#include <vector>
using namespace std;
 
typedef long long ll;
typedef long double LD;
#define int ll
#define double LD
#define pb push_back
#define mp make_pair
#define REP(i,n) for (int i = 0; i < n; i++)
#define loop(i,n) for (int i = 0; i < n; i++)
#define FOR(i,a,b) for (int i = a; i < b; i++)
#define REPD(i,n) for (int i = n-1; i >= 0; i--)
#define FORD(i,a,b) for (int i = a; i >= b; i--)
#define remax(a,b) a = max(a,b)
#define remin(a,b) a = min(a,b)
#define mem(a, b) memset(a, b, sizeof a)
#define append push_back
#define all(v) v.begin(),v.end()
#define F first
#define S second
#define print(x) cout << (x) << endl
#define PRECISION(x) cout << fixed << setprecision(x)
#define FAST_IO ios::sync_with_stdio(0); cin.tie(0); cout.tie(0)

const double PI=acos(-1);
const int MOD = 1000000007;
const int FMOD = 998244353;
const double eps = 1e-9;

mt19937 RNG(chrono::steady_clock::now().time_since_epoch().count()); 
#define SHUF(v) shuffle(all(v), RNG);
// Use mt19937_64 for 64 bit random numbers.
 
void solve();
 
signed main(){
    FAST_IO;
    PRECISION(10);
 
    int t = 1;
    //cin>> t;
    FOR(testcase,1,t+1){
        // printf("Case #%lld: ",testcase); 
        solve();
    }
    cerr << "\nTime elapsed: " << 1000 * clock() / CLOCKS_PER_SEC << "ms\n";
}

const int MAXN = 20;

int n, m;
vector<string> grid;
int dist[MAXN][MAXN], vis[MAXN][MAXN];
int dx[] = {-1, 1, 0, 0}, dy[] = {0, 0, 1, -1};

bool valid(int x, int y){
    return (0 <= x and x < n and 0 <= y and y < m and grid[x][y] == '.');
}

int bfs(int rootx, int rooty){
    queue<pair<int, int> > q;

    for(int i = 0; i < n; i++)
        for(int j = 0; j < m; j++){
            dist[i][j] = 10000;
            vis[i][j] = 0;
        }

    vis[i][j] = 1;
    dist[rootx][rooty] = 0;
    q.push(mp(rootx, rooty));

    while(!q.empty()){
        auto node = q.front();
        q.pop();
        for(int i = 0; i < 4; i++){
            int newx = node.F + dx[i], newy = node.S + dy[i];
            if(valid(newx, newy) and !vis[newx][newy]){
                vis[newx][newy] = 1;
                dist[newx][newy] = 1 + dist[node.F][node.S];
                q.push(mp(newx, newy));
            }
        }
    }

    int ret = 0;
    for(int i = 0; i < n; i++)
        for(int j = 0; j < m; j++)
            if(valid(i, j))
                ret = max(ret, dist[i][j]);

    return ret;
}   

void solve(){
    cin>> n >> m;
    for(int i = 0; i < n; i++){
        string s;
        cin>> s;
        grid.pb(s);
    }
    int ans = 0;
    for(int i = 0; i < n; i++)
        for(int j = 0; j < m; j++)
            if(valid(i, j))
                ans = max(ans, bfs(i, j));
    print(ans);
}

Compiler output:
a.cc: In function 'll bfs(ll, ll)':
a.cc:103:9: error: 'i' was not declared in this scope
  103 |     vis[i][j] = 1;
      |         ^
a.cc:103:12: error: 'j' was not declared in this scope
  103 |     vis[i][j] = 1;
      |            ^