写出Dijkstra算法代码

以下是Python实现的Dijkstra算法代码：

import heapq

def dijkstra(graph, start):
    distances = {node: float('inf') for node in graph}
    distances[start] = 0
    pq = [(0, start)]
    visited = set()

    while pq:
        (distance, node) = heapq.heappop(pq)

        if node in visited:
            continue

        visited.add(node)

        for neighbor, weight in graph[node].items():
            distance_to_neighbor = distance + weight
            if distance_to_neighbor < distances[neighbor]:
                distances[neighbor] = distance_to_neighbor
                heapq.heappush(pq, (distance_to_neighbor, neighbor))

    return distances

其中，graph是一个字典，表示有向图，每个节点对应一个字典，存储它的邻居节点及边权重。例如：

graph = {
    'A': {'B': 2, 'C': 4},
    'B': {'C': 1, 'D': 4},
    'C': {'D': 1},
    'D': {}
}

start表示起点。算法返回一个字典distances，表示从起点到每个节点的最短距离。