class Solution:
    def orangesRotting(self, grid: List[List[int]]) -> int:
        if not grid or not grid[0]:
            return 0 # no orange

        h, w = len(grid), len(grid[0])
        fresh_oranges = 0
        curr_rotten_oranges = []
        max_steps = 0

        for i in range(h):
            for j in range(w):
                if grid[i][j] == 1:
                    fresh_oranges += 1
                elif grid[i][j] == 2:
                    curr_rotten_oranges.append((i, j))

        def valid_fresh_orange(i, j):
            return 0 <= i < h and 0 <= j < w and grid[i][j] == 1

        directions = [(1,0),(-1,0),(0,1),(0,-1)]
        while fresh_oranges > 0 and curr_rotten_oranges:
            next_rotten_oranges = []

            for i, j in curr_rotten_oranges:
                for dx, dy in directions:
                    if valid_fresh_orange(i+dx, j+dy):
                        grid[i+dx][j+dy] = 2
                        fresh_oranges -= 1
                        next_rotten_oranges.append((i+dx, j+dy))

            curr_rotten_oranges = next_rotten_oranges
            max_steps += 1


        return -1 if fresh_oranges else max_steps