有向加权图(权值存在负数)求最短路径的边介数
时间: 2024-06-06 10:10:41 浏览: 19
最短路径的边介数可以通过 Floyd 算法来计算。具体步骤如下:
1. 初始化边介数矩阵为 0。
2. 对于每一对节点 u 和 v,用 Floyd 算法求出它们之间的最短路径长度和路径条数。
3. 对于每一对节点 u 和 v,遍历它们之间的所有路径,如果路径上的边 e 在最短路径中出现了 k 次,则将边介数矩阵中 e 的值加上 1/k。
4. 边介数矩阵中的值即为最短路径的边介数。
需要注意的是,当存在负环时,最短路径长度没有意义,因为可以无限次地绕着负环走来缩小路径长度。此时最短路径的边介数也没有意义。
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有向加权图(权值存在负数)求最短路径的边介数python实现
以下是Python实现有向加权图最短路径的边介数的示例代码:
```python
import heapq
def shortest_path_edge_betweenness(G, s, t):
# 初始化边介数和距离
betweenness = {(u, v): 0.0 for u in G for v in G[u]}
distance = {u: float('inf') for u in G}
distance[s] = 0.0
paths = {u: [] for u in G}
paths[s] = [[s]]
# 计算最短路径和路径数量
queue = [(0.0, s, None)]
while queue:
(dist, u, p) = heapq.heappop(queue)
for v, e in G[u].items():
w = e['weight']
if distance[v] > dist + w:
distance[v] = dist + w
paths[v] = [path + [v] for path in paths[u]]
heapq.heappush(queue, (distance[v], v, u))
elif distance[v] == dist + w:
paths[v].extend([path + [v] for path in paths[u]])
# 计算边介数
credit = {u: 1.0 for u in G}
while paths[t]:
path = paths[t].pop()
for i in range(len(path) - 1):
u, v = path[i], path[i + 1]
credit[v] += credit[u] * betweenness[(u, v)]
betweenness[(u, v)] += credit[u] / credit[v]
return betweenness
```
该算法使用Dijkstra算法计算最短路径和路径数量,并通过迭代计算边介数。该算法的时间复杂度为$O(mn\log n)$,其中$m$是边数,$n$是节点数。
拓扑图的加权(权值有负值)最短路径以及边介数的计算python
拓扑图的加权最短路径可以使用Dijkstra算法或Bellman-Ford算法实现,而边介数可以使用Brandes算法实现。
以下是使用Python实现Dijkstra算法和Brandes算法的示例代码:
```python
# Dijkstra算法
def dijkstra(graph, start):
dist = {node: float('inf') for node in graph} # 起点到各点的距离,默认无限大
dist[start] = 0 # 起点到自身距离为0
visited = set() # 已访问的节点
while len(visited) < len(graph):
node = min(set(dist.keys()) - visited, key=dist.get) # 未访问的节点中距离最短的节点
visited.add(node)
for neighbor, weight in graph[node].items():
newdist = dist[node] + weight
if newdist < dist[neighbor]:
dist[neighbor] = newdist
return dist
# Brandes算法
def brandes(graph):
betweenness = {node: 0.0 for node in graph} # 节点介数初始化为0
for node in graph:
stack = []
pred = {v: [] for v in graph}
sigma = {v: 0.0 for v in graph}
sigma[node] = 1.0
dist = {v: -1 for v in graph}
dist[node] = 0
queue = [node]
while queue:
v = queue.pop(0)
stack.append(v)
for w in graph[v]:
if dist[w] < 0:
queue.append(w)
dist[w] = dist[v] + 1
if dist[w] == dist[v] + 1:
sigma[w] += sigma[v]
pred[w].append(v)
delta = {v: 0.0 for v in graph}
while stack:
w = stack.pop()
for v in pred[w]:
delta[v] += (sigma[v]/sigma[w]) * (1 + delta[w])
if w != node:
betweenness[w] += delta[w]
return betweenness
```
其中,graph是一个字典,表示拓扑图的邻接矩阵,例如:
```python
graph = {
'A': {'B': 1, 'C': 4},
'B': {'D': 2},
'C': {'D': 3},
'D': {}
}
```
表示图中有4个节点A、B、C、D,边AB权值为1,AC权值为4,BD权值为2,CD权值为3。注意,如果两个节点之间没有边,则对应的权值为空字典。