Matrix & Grid Traversal

from collections import deque

def oranges_rotting(grid: list[list[int]]) -> int:
    ROWS, COLS = len(grid), len(grid[0])
    queue = deque()
    fresh_count = 0
    
    # 1. Multi-source initialization: find all rotten oranges and count fresh
    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_count += 1
                
    if fresh_count == 0:
        return 0
        
    minutes = 0
    DIRECTIONS = [(0, 1), (0, -1), (1, 0), (-1, 0)]
    
    # 2. BFS spreading level by level
    while queue and fresh_count > 0:
        minutes += 1
        for _ in range(len(queue)):
            r, c = 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  # Infect fresh orange
                    fresh_count -= 1
                    queue.append((nr, nc))
                    
    return minutes if fresh_count == 0 else -1