Bellman-Ford算法代码

下面是Bellman-Ford算法的Python代码：

def bellman_ford(graph, start_node):
    # 初始化距离为无穷大
    distance = {node: float('inf') for node in graph}
    # 起始节点距离为0
    distance[start_node] = 0
    
    # 遍历所有节点，最多进行n-1次
    for _ in range(len(graph) - 1):
        # 遍历所有边，更新距离
        for node in graph:
            for neighbor in graph[node]:
                new_distance = distance[node] + graph[node][neighbor]
                if new_distance < distance[neighbor]:
                    distance[neighbor] = new_distance
    
    # 检查是否存在负权环
    for node in graph:
        for neighbor in graph[node]:
            if distance[node] + graph[node][neighbor] < distance[neighbor]:
                raise ValueError("存在负权环")
    
    return distance

其中，graph是一个字典，表示图的邻接表。start_node是起始节点的名称，distance是一个字典，表示每个节点到起始节点的最短距离。如果存在负权环，则抛出ValueError异常。