Update BFS impls
I've subtly been implementing breadth-first traversals in graphs incorrectly. The change is subtle, but updating `seen` needs to happen immediately after queuing an item. The results will remain the same, but the runtimes will differ dramatically. I didn't notice this until I attempted to complete LeetCode's "count islands" challenge, and LeetCode rejected my solution because it could not finish before timing out. After looking at other candidates' solutions and comparing them to mine, I couldn't see any difference... except for this subtle difference. This SO answer provides a helpful explanation: https://stackoverflow.com/questions/45623722/marking-node-as-visited-on-bfs-when-dequeuing The take-away lesson here is to always call `seen.add(..)` immediately after enqueuing.
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2 changed files with 3 additions and 3 deletions
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@ -8,6 +8,7 @@ def maybe_queue(row, col, game, q, seen):
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if row >= 0 and row < len(game) and col >= 0 and col < len(game[0]):
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if game[row][col] == 'L' and (row, col) not in seen:
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q.append((row, col))
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seen.add((row, col))
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def visit_island(row, col, game, seen):
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"""
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@ -18,7 +19,6 @@ def visit_island(row, col, game, seen):
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q.append((row, col))
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while q:
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row, col = q.popleft()
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seen.add((row, col))
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maybe_queue(row - 1, col, game, q, seen) # UP
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maybe_queue(row + 1, col, game, q, seen) # DOWN
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maybe_queue(row, col - 1, game, q, seen) # LEFT
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@ -25,10 +25,10 @@ class GraphNode(object):
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while xs:
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node = xs.popleft()
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result.append('{} ({})'.format(node.label, str(node.color)))
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seen.add(node.label)
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for c in node.neighbors:
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if c.label not in seen:
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xs.append(c)
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seen.add(node.label)
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return ', '.join(result)
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def color_graph(graph, d):
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@ -40,11 +40,11 @@ def color_graph(graph, d):
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while xs:
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x, color = xs.popleft()
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x.color = color
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seen.add(x.label)
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for c in x.neighbors:
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if c.label not in seen:
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palette.advance()
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xs.append((c, palette.get()))
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seen.add(x.label)
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a = GraphNode('a')
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b = GraphNode('b')
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