Rotting Oranges – Solution & Complexity

def oranges_rotting(grid: list[list[int]]) -> int:
    rows, cols = len(grid), len(grid[0]); queue = []; fresh = 0
    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
    head = minutes = 0
    while head < len(queue) and fresh:
        end = len(queue)
        while head < end:
            r, c = queue[head]; head += 1
            for dr, dc in ((1,0),(-1,0),(0,1),(0,-1)):
                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; queue.append((nr, nc))
        minutes += 1
    return minutes if fresh == 0 else -1