Python Language Deque Module Breadth First Search


Example

The Deque is the only Python data structure with fast Queue operations. (Note queue.Queue isn't normally suitable, since it's meant for communication between threads.) A basic use case of a Queue is the breadth first search.

from collections import deque

def bfs(graph, root):
    distances = {}
    distances[root] = 0
    q = deque([root])
    while q:
        # The oldest seen (but not yet visited) node will be the left most one.
        current = q.popleft()
        for neighbor in graph[current]:
            if neighbor not in distances:
                distances[neighbor] = distances[current] + 1
                # When we see a new node, we add it to the right side of the queue.
                q.append(neighbor)
    return distances

Say we have a simple directed graph:

graph = {1:[2,3], 2:[4], 3:[4,5], 4:[3,5], 5:[]}

We can now find the distances from some starting position:

>>> bfs(graph, 1)
{1: 0, 2: 1, 3: 1, 4: 2, 5: 2}

>>> bfs(graph, 3)
{3: 0, 4: 1, 5: 1}