function updateMatrix(mat) {
  const m = mat.length, n = mat[0].length;
  const dist = Array.from({length: m}, () => new Array(n).fill(Infinity));
  const queue = [];
  for (let i = 0; i < m; i++)
    for (let j = 0; j < n; j++)
      if (mat[i][j] === 0) { dist[i][j] = 0; queue.push([i, j]); }
  const dirs = [[1,0],[-1,0],[0,1],[0,-1]];
  let idx = 0;
  while (idx < queue.length) {
    const [r, c] = queue[idx++];
    for (const [dr, dc] of dirs) {
      const nr = r + dr, nc = c + dc;
      if (nr >= 0 && nr < m && nc >= 0 && nc < n && dist[nr][nc] > dist[r][c] + 1) {
        dist[nr][nc] = dist[r][c] + 1;
        queue.push([nr, nc]);
      }
    }
  }
  return dist;
}