在嵌套的 map 中输入值

Question

I was thinking of ways to implement an adjacency list for a graph and thought a nested map might be a good idea(because of the fast lookup times) for the outer map I can say map.put but for the inner map which is unnamed how我是否将值放入

    Scanner sc = new Scanner(System.in);
    Map<Integer,Map<Integer,Integer>> graph = new HashMap<>();
    int V = sc.nextInt();
    int E = sc.nextInt();
    for(int i=0;i<E;i++){
        int v1 = sc.nextInt();
        int v2 = sc.nextInt();
        int w = sc.nextInt();
        graph.put(v1, Map.entry(v2, w));
        graph.put(v2, Map.entry(v1, w));

    }

其中 v1 和 v2 是图的节点/顶点，w 是两个顶点之间的权重/边

有人有什么主意吗？ 提前致谢：）

Answer 1

您可以像这样放入图表

if (!graph.contains(v1)) {
    grah.put(v1, new HashMap<>());
}

if (!graph.contains(v2)) {
    grah.put(v2, new HashMap<>());
}

graph.get(v1).put(v2, w);
graph.get(v1).put(v2, w);

Answer 2

1. 如果不存在，则创建内部 map graph.putIfAbsent(v1, new HashMap<>());
2. 获取内部 map graph.get(v1)
3. 将值放入内部 map graph.get(v1).put(v2, 2)

graph.putIfAbsent(v1, new HashMap<>());
graph.putIfAbsent(v2, new HashMap<>());
graph.get(v1).put(v2, w);
graph.get(v2).put(v1, w);

Answer 3

我不确定你从哪里得到 Map.entry 方法，但通常你会像外部一样创建内部 map 然后把它放进去。像：

Scanner sc = new Scanner(System.in);
Map<Integer,Map<Integer,Integer>> graph = new HashMap<>();
int V = sc.nextInt();
int E = sc.nextInt();
for(int i=0;i<E;i++){
    int v1 = sc.nextInt();
    int v2 = sc.nextInt();
    int w = sc.nextInt();

    Map<Integer,Integer> innerMap = new HashMap<>();
    innerMap.put(v2, w);
    graph.put(v1, innerMap);

    Map<Integer,Integer> andAnotherMap = new HashMap<>();
    andAnotherMap.put(v2, w);
    graph.put(v2, andAnotherMap);

}

当然，将它包装在一个方法中会更干净，所以你最终会得到：

private Map<Integer, Integer> innerMap(int a, int b) {
  Map<Integer,Integer> innerMap = new HashMap<>();
  innerMap.put(a, b);
  return innerMap;
}

...

Scanner sc = new Scanner(System.in);
Map<Integer,Map<Integer,Integer>> graph = new HashMap<>();
int V = sc.nextInt();
int E = sc.nextInt();
for(int i=0;i<E;i++){
    int v1 = sc.nextInt();
    int v2 = sc.nextInt();
    int w = sc.nextInt();
    graph.put(v1, innerMap(v2, w));
    graph.put(v2, innerMap(v1, w));

}

希望这可以帮助！