MAXR _BFS (Idea from Nguyen Nghia - Kien Giang TSTer)

#define task "MAXR"
#include <iostream>
#include <cstdio>
#include <queue>

using namespace std;

using ll = long long;
using ld = long double;
template <class T>
void readInt(T &x)
{
    x = 0;
    register int c;
    while ((c = getchar()) && (c > '9' || c < '0'))
        ;
    for (; c >= '0' && c <= '9'; c = getchar())
        x = x * 10 + c - '0';
}

constexpr int N = 5e3 + 5;
constexpr int Inf = 1e9 + 7;
int x[] = {0, 0, -1, 1},
    y[] = {-1, 1, 0, 0};
int m, n;
int a[N][N], d[N][N];

void Read()
{
    // cin >> m >> n;
    readInt(m), readInt(n);

    for (int i = 1; i <= m; ++i)
        for (int j = 1; j <= n; ++j)
            // cin >> a[i][j];
            readInt(a[i][j]);
}

void Solve()
{
    fill_n(&d[0][0], N * N, Inf);

    queue<pair<int, int>> q;

    for (int i = 0; i <= m + 1; ++i)
        for (int j = 0; j <= n + 1; ++j)
            if (a[i][j] == 0)
            {
                d[i][j] = 0;
                q.emplace(i, j);
            }

    int ans(0);

    while (!q.empty())
    {
        pair<int, int> c = q.front();
        q.pop();

        ans = max(ans, d[c.first][c.second] - 1);

        for (int i = 0; i < 4; ++i)
        {
            int u = c.first + x[i],
                v = c.second + y[i];

            if (u > 0 && u <= m && v > 0 && v <= n && d[u][v] == Inf)
            {
                d[u][v] = d[c.first][c.second] + 1;
                q.emplace(u, v);
            }
        }
    }

    cout << ans;
}

int32_t main()
{
    ios_base::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    if (fopen(task ".INP", "r"))
    {
        freopen(task ".INP", "r", stdin);
        freopen(task ".OUT", "w", stdout);
    }

    Read();
    Solve();
}