#include <stdio.h>
#include <stdlib.h>

#define not !
#define and &&
#define or ||

#define ri register int
#define rep(inc, frm, to) for (ri inc = frm; inc < (to); ++inc)
#define rep2(inc, frm, to) for (ri inc = frm; inc > (to); --inc)

typedef long long int ll;
#define r() fast_read()

#define MAX_N 1010

int is_digit(const char c) {
  return c >= 48 and c <= 57;
}

ll fast_read(void) {
  ll n = 0, sign = 1;
  char c = getchar();
  while (not is_digit(c)) {
    if (c == '-') sign = ~0;
    c = getchar();
  }
  while (is_digit(c)) {
    n = (n << 1) + (n << 3) + (c ^ 48);
    c = getchar();
  }
  return sign * n;
}

typedef struct Coordinate Coord;
struct Coordinate {
  int x, y;
} q[MAX_N * MAX_N];
int front, rear;


void print_board(int** board, const size_t m, const size_t n) {
  rep(y, 0, m) rep(x, 0, n) printf("%d%c", *(*(board + y) + x), x == n - 1 ? 10 : 32);
}

void BFS(int** board, const size_t m, const size_t n) {
  const int dirs[] = {0, -1, 0, 1, 0};

  int steps = 0;
  while (front < rear) {
    size_t s = rear - front;
    while (s--) {
      const int x = (q + front)->x, y = (q + front)->y; ++front;
      rep(d, 0, 4) {
        const int nx = x + *(dirs + d), ny = y + *(dirs + d + 1);
        if (not(nx >= 0 and nx < n and ny >= 0 and ny < m and *(*(board + ny) + nx) < 0))
          continue;
        *(q + rear++) = (Coord) {nx, ny};
        *(*(board + ny) + nx) = steps + 1;
      }

    }
    ++steps;
  }
}

signed main(int argc, char const *argv[]) {
  // freopen("test.in", "r", stdin);
  int m = r(), n = r(), board[MAX_N][MAX_N];

  char s[MAX_N];
  rep(y, 0, m) {
    scanf("%s", s);
    rep(x, 0, n) {
      *(*(board + y) + x) = *(s + x) - 49;
      if (!*(*(board + y) + x)) *(q + rear++) = (Coord) {x, y};
    }
  }

  int* rows_of_board[m];
  rep(y, 0, m)
    *(rows_of_board + y) = *(board + y);

  BFS(rows_of_board, m, n);
  print_board(rows_of_board, m, n);
  // fclose(stdin);
  return ~~(0 ^ 0);
}

BFS Template 和#173题一样 一行代码都没改

#include <iostream>
#include <cstring>
#include <queue>
#include <vector>

using namespace std;

const int N = 1010;
int g[N][N], visited[N][N], m, n;

int main() {
  cin >> m >> n;

  string line;
  for (int i = 0; i < m; ++i) {
    cin >> line;
    for (int j = 0; j < n; ++j) g[i][j] = line[j] - '0';
  }

  memset(visited, 0, sizeof visited);

  queue<pair<int, int>> q;
  for (int y = 0; y < m; ++y)
    for (int x = 0; x < n; ++x)
      if (g[y][x] == 1) {
        q.emplace(x, y);
        visited[y][x] = 1;
      }

  static constexpr int dirs[] { 0, -1, 0, 1, 0};

  int steps = 0;
  while (not q.empty()) {
    size_t sz = q.size();
    while (sz--) {
      const auto [x, y] = q.front(); q.pop();
      g[y][x] = steps;
      for (int i = 0; i < 4; ++i) {
        int nx = x + dirs[i], ny = y + dirs[i + 1];
        if (nx < 0 || ny < 0 || nx == n || ny == m || visited[ny][nx])
          continue;
        q.emplace(nx, ny);
        visited[ny][nx] = 1; // mark as visted
      }
    }
    ++steps;
  }

  for (int y = 0; y < m; ++y) {
    for (int x = 0; x < n; ++x) printf("%d ", g[y][x]);
    printf("\n");
  }

  return 0;
}