solution.py

from collections import deque

class Solution:
    def orangesRot(self, mat):
        n, m = len(mat), len(mat[0])
        q = deque()
        fresh = 0
        
        # Step 1
        for i in range(n):
            for j in range(m):
                if mat[i][j] == 2:
                    q.append((i, j))
                elif mat[i][j] == 1:
                    fresh += 1
        
        directions = [(-1,0),(1,0),(0,-1),(0,1)]
        time = 0
        
        # Step 2: BFS
        while q and fresh > 0:
            for _ in range(len(q)):
                x, y = q.popleft()
                
                for dx, dy in directions:
                    nx, ny = x + dx, y + dy
                    
                    if 0 <= nx < n and 0 <= ny < m and mat[nx][ny] == 1:
                        mat[nx][ny] = 2
                        q.append((nx, ny))
                        fresh -= 1
            time += 1
        
        return time if fresh == 0 else -1