Rotting Oranges

def orangesRotting(grid):
    rows, cols = len(grid), len(grid[0])
    minutes = 0
    while True:
        # Find every fresh orange that touches a rotten one right now.
        to_rot = []
        for r in range(rows):
            for c in range(cols):
                if grid[r][c] != 1:
                    continue
                for nr, nc in ((r + 1, c), (r - 1, c), (r, c + 1), (r, c - 1)):
                    if 0 <= nr < rows and 0 <= nc < cols and grid[nr][nc] == 2:
                        to_rot.append((r, c))
                        break
        if not to_rot:
            break
        # Rot them only after the scan, so the rot moves one step a minute.
        for r, c in to_rot:
            grid[r][c] = 2
        minutes += 1
    for row in grid:
        if 1 in row:
            return -1
    return minutes

from collections import deque


def orangesRotting(grid):
    rows, cols = len(grid), len(grid[0])
    queue = deque()
    fresh = 0
    # Every orange that is rotten at minute 0 starts in the queue.
    for r in range(rows):
        for c in range(cols):
            if grid[r][c] == 2:
                queue.append((r, c))
            elif grid[r][c] == 1:
                fresh += 1
    minutes = 0
    while queue and fresh > 0:
        minutes += 1
        # The queue holds exactly the oranges that went rotten last minute.
        # Rot their fresh neighbours; those become the next minute's queue.
        for _ in range(len(queue)):
            r, c = queue.popleft()
            for nr, nc in ((r + 1, c), (r - 1, c), (r, c + 1), (r, c - 1)):
                if 0 <= nr < rows and 0 <= nc < cols and grid[nr][nc] == 1:
                    grid[nr][nc] = 2
                    fresh -= 1
                    queue.append((nr, nc))
    return minutes if fresh == 0 else -1