python - 广度优先搜索算法

Question

就像我之前遇到的一个问题一样，我正在尝试创建一个广度优先搜索算法，该算法采用图形并输出顶点访问顺序。它需要一个邻接矩阵(表示图形)作为输入，这是我目前所拥有的。

import sys
import Queue

# Input has to be adjacency matrix or list
graphAL2 = {0 : [1,2,3],
        1 : [0,3,4],
        2 : [0,4,5],
        3 : [0,1,5],
        4 : [1,2],
        5 : [2,3] }

# NEED TO FIX:
# - Final graphAL2v print is only displaying key values as 1, not iterating
# through graph and visiting each vertex

def main():
    count = 0
    graphAL2v = {}

    for key, value in graphAL2.items():
        graphAL2v[key] = 0

    print(graphAL2v)

    for key in graphAL2v: # each vertex v in V
        if graphAL2v[key] == 0: # is marked with 0
            bfs(key, count, graphAL2, graphAL2v)
    print(graphAL2v)

def bfs(v, count, graphal, graphv):
    count = count + 1
    print('Visiting', v)

    # Mark v with count and initialize queue with v
    graphv[v] = count
    visited = Queue.Queue()

    while not visited.empty(): #queue not empty:
        print('queue is not empty')
        for element in graphal[v]: # each vertex w in V adjacent to front vertex
            if element == 0:
                count = count + 1
                # mark w with count
                graphal[v] = count
                visited.put()
        visited.get()

if __name__ == '__main__':
    sys.exit(main())

我遇到的问题是我的输出

{0: 0, 1: 0, 2: 0, 3: 0, 4: 0, 5: 0}
('Visiting', 0)
('Visiting', 1)
('Visiting', 2)
('Visiting', 3)
('Visiting', 4)
('Visiting', 5)
{0: 1, 1: 1, 2: 1, 3: 1, 4: 1, 5: 1}

将列表中所有顶点的访问顺序显示为 1，而当它遍历“图形”时应该将每个顶点的访问顺序显示为不同的数字。我相信这个错误源于 bfs() 函数的 while 循环。有关尝试修复代码以便获得所需输出的任何建议吗？我对 Python 中的队列也不太熟悉，因此不胜感激。

Answer 1

您的代码中有很多问题 -

1. 首先，您永远不会将任何东西放入您创建的队列中，因此它始终是空的，您需要将 v 放入队列中，然后放在 while 循环，即起点。

2. 其次，在 for 循环中，您正在检查 element == 0 ，这是错误的，您需要检查 graphv[element ] == 0 ，即元素是否已经被访问过。

3. 第三，在for循环中，你需要设置graphv[element] = count，这表示你已经激活了element .

4. 您没有使用 visited.put() 将任何内容放入队列中，您需要将要放入队列中的元素作为参数传递。

5. 从Queue取回元素时，需要将其赋值回v，否则v永远不会改变，v 表示正在迭代的当前元素。

示例代码-

import sys
import Queue

# Input has to be adjacency matrix or list
graphAL2 = {0 : [1,2,3],
        1 : [0,3,4],
        2 : [0,4,5],
        3 : [0,1,5],
        4 : [1,2],
        5 : [2,3] }

# NEED TO FIX:
# - Final graphAL2v print is only displaying key values as 1, not iterating
# through graph and visiting each vertex

def main():
    count = 0
    graphAL2v = {}

    for key, value in graphAL2.items():
        graphAL2v[key] = 0

    print(graphAL2v)

    for key in graphAL2v: # each vertex v in V
        if graphAL2v[key] == 0: # is marked with 0
            bfs(key, count, graphAL2, graphAL2v)
    print(graphAL2v)

def bfs(v, count, graphal, graphv):
    count = count + 1
    print('Visiting', v)

    # Mark v with count and initialize queue with v
    graphv[v] = count
    visited = Queue.Queue()
    visited.put(v)
    while not visited.empty(): #queue not empty:
        print('queue is not empty')
        for element in graphal[v]: # each vertex w in V adjacent to front vertex
            if graphv[element] == 0:
                count = count + 1
                # mark w with count
                graphv[element] = count
                visited.put(element)
        v = visited.get()
    return count

if __name__ == '__main__':
    sys.exit(main())

---

演示(经过上述更改后)-

{0: 0, 1: 0, 2: 0, 3: 0, 4: 0, 5: 0}
Visiting 0
queue is not empty
queue is not empty
queue is not empty
queue is not empty
queue is not empty
queue is not empty
{0: 1, 1: 2, 2: 3, 3: 4, 4: 5, 5: 6}