994. Rotting Oranges - LeetCode Solution

class Solution:
    def orangesRotting(self, grid: List[List[int]]) -> int:
        nb_rows = len(grid)
        nb_cols = len(grid[0])
        x = [0, 0, 1, -1]
        y = [1, -1, 0, 0]
        t = 0
        queue = []
        queue2 = []

        grid_set = set([num for l in grid for num in l])
        if 1 in grid_set and 2 not in grid_set:
            return -1
        if 1 not in grid_set and 2 in grid_set:
            return 0

        for i in range(nb_rows):
            for j in range(nb_cols):
                if grid[i][j] == 2:
                    queue.append((i, j))

        while queue:
            (i, j) = queue.pop(0)
            for k in range(4):
                new_x = i + x[k]
                new_y = j + y[k]
                if new_x >= 0 and new_x < nb_rows and new_y >= 0 and new_y < nb_cols:
                    if grid[new_x][new_y] == 1:
                        grid[new_x][new_y] = 2
                        queue2.append((new_x, new_y))
            if not queue:
                if queue2:
                    t += 1
                    queue = queue2
                    queue2 = []

        grid_set = set([num for l in grid for num in l])
        if 1 in grid_set:
            return -1
        return t