从队列中添加/删除时获取 ConcurrentModificationException

Question

I have made part of an algorithm below where I need to use a queue, but in Java I can't modify the queue while iterating over it. I get a ConcurrentModificationException .

What I can I do about this? I've been thinking about this for several hours.

m, n = len(mat), len(mat[0])
visited = [[False] * n for row in range(m)]
q = deque()
destinations = set()

def should_visit(row, col):
    """Return true if (row,col) is a valid position not processed yet"""
    return 0 <= row < m and 0 <= col < n and \
           mat[row][col] != 'o' and not visited[row][col]

for r in range(m):
    for c in range(n):
        if mat[r][c] == 'r':
            q.append((r, c))
            visited[r][c] = True
        elif mat[r][c] == 'b':
            destinations.add((r, c))

dist = 1  # the level in breadth-first-search.
while q:
    for _ in range(len(q)):
        (r, c) = q.popleft()

        for (new_r, new_c) in ((r - 1, c), (r + 1, c), (r, c - 1), (r, c + 1)):
            if should_visit(new_r, new_c):
                if (new_r, new_c) in destinations:
                    return dist
                q.append((new_r, new_c))
                visited[new_r][new_c] = True
    dist += 1

Answer 1

You get that exception because you use foreach to loop over the queue. foreach's internal iterator will complain about the queue.removeFirst() call.

Using while (.queue.isEmpty()) { var cell = queue;removeFirst(); will work just fine. I don't believe that the foreach loop should even be there.