from collections import deque

def oranges_rotting(grid: list[list[int]]) -> int:
    rows, cols = len(grid), len(grid[0])
    queue = deque()
    fresh = 0

    # find all rotten oranges and count fresh ones
    for r in range(rows):
        for c in range(cols):
            if grid[r][c] == 2:
                queue.append((r, c, 0))  # (row, col, time)
            elif grid[r][c] == 1:
                fresh += 1

    if fresh == 0:
        return 0

    directions = [(0,1), (0,-1), (1,0), (-1,0)]
    max_time = 0

    while queue:
        r, c, time = queue.popleft()
        for dr, dc in directions:
            nr, nc = r + dr, c + dc
            if 0 <= nr < rows and 0 <= nc < cols and grid[nr][nc] == 1:
                grid[nr][nc] = 2
                fresh -= 1
                max_time = max(max_time, time + 1)
                queue.append((nr, nc, time + 1))

    return max_time if fresh == 0 else -1

# Time: O(rows * cols), Space: O(rows * cols)

Stacks & Queues — Monotonic Stack, BFS

Stacks & Queues — Monotonic Stack, BFS

Stacks & Queues — Monotonic Stack, BFS