title: Leetcode Problem 994- rotting oranges solution won't work :(
body: <p>So I've come across this problem while trying to enhance my skills. I watched the Neetcode solution video to it 2 days ago, where he goes over a detailed solution, and I decided to tackle it myself today since I didn't have his code fresh on my mind, but I still had the intuition and hence, I thought I could maybe be able to solve it on my own, but oh how I was wrong..... After SEVERAL errors thrown that I solved, I got stuck with this error <strong>RuntimeError: deque mutated during iteration</strong> which I don't understand. I went through the logic and the solution makes sense to me &amp; I don't understand why it won't budge. Watching a solution video and not being able to come to one myself due to some dumb runtime error makes me feel extremely inadequate about my skills and essential makes me feel like garbage. I put a lot of effort into making this solution, yet it just won't damn run. If anyone could help me out, I would GREATLY appreciate that. Im very hopeless right now.</p>
  <pre><code>from collections import deque
  
  def orangesRotting(grid):
      time = 0
      row, col = len(grid), len(grid[0])
      rotten = deque()
      fresh = 0
      isrotting = []
      for r in range(row):
          for c in range(col):
              if grid[r][c] == 1:
                  fresh += 1
              if grid[r][c] == 2:
                  rotten.append((r,c))
      def helper(r, c, fresh):
          if r &lt; 0 or r == row or c &lt; 0 or c == col or grid[r][c] != 1:
              return
          grid[r][c] = 2
          fresh -= 1
          isrotting.append((r,c))
      while rotten or fresh &gt; 0:
          for (ri, ci) in rotten:
              helper(ri, ci + 1, fresh)
              helper(ri, ci - 1, fresh)
              helper(ri + 1, ci, fresh)
              helper(ri - 1, ci, fresh)
              rotten.pop()
              if (ri, ci) in isrotting:
                  isrotting.remove((ri, ci))
          for (rrtn, crtn) in isrotting:
              rotten.append((rrtn, crtn))
          time += 1
      if not rotten and fresh &gt; 0:
          time = -1
      return time
     
  grid = [[2,1,1],[1,1,0],[0,1,1]]
  print(orangesRotting(grid))
  </code></pre>