java - 如何反转 HashMap 的键和值？

Question

我创建了一个HashMap<String,List<Integer>>. HashMap<Integer,List<String>>现在我想通过替换第一个映射中的键和值来创建反向。

例如，

原始哈希图：{ A=[2,1], B=[1,3,4], C=[5], D=[3], E=[2,4] }

反向哈希映射：{ 1=[A,B], 2=[A,E], 3=[B,D], 4=[B,E], 5=[C] }

Answer 1

实现以下伪代码：

Parameter: original map OM

Let RM be an empty map.
For all String S in OM:
    For all integer I in the OM[S]:
        If RM do not contains I:
            Let RM[I] be an empty list.
        Add S to RM[I].
Return RM.

Answer 2

    HashMap<String, List<Integer>> hMap=new HashMap<String, List<Integer>>();
    hMap.put("A",new ArrayList<Integer>(Arrays.asList(2,1)));
    hMap.put("B",new ArrayList<Integer>(Arrays.asList(1,3,4)));
    hMap.put("C",new ArrayList<Integer>(Arrays.asList(5)));
    hMap.put("D",new ArrayList<Integer>(Arrays.asList(3)));
    hMap.put("E",new ArrayList<Integer>(Arrays.asList(2,4)));

    //Original HashMap: { A=[2,1], B=[1,3,4], C=[5], D=[3], E=[2,4] }

    //Reversed HashMap: { 1=[A,B], 2=[A,E], 3=[B,D], 4=[B,E], 5=[C] }

    HashMap<Integer,List<String>> result = new HashMap<>(hMap.size());
    for(Map.Entry<String,List<Integer>> entry : hMap.entrySet()) {
        for(Integer n : entry.getValue()) {             
            if(!result.containsKey(n)) { 
                result.put(n,new ArrayList<String>());
            }
            result.get(n).add(entry.getKey());
        }
    }

    System.out.println(hMap);
    System.out.println(result);

输出

{D=[3], E=[2, 4], A=[2, 1], B=[1, 3, 4], C=[5]}

{1=[A, B], 2=[E, A], 3=[D, B], 4=[E, B], 5=[C]}

Answer 3

尝试这个。

HashMap<Integer,List<String>> reverse = new HashMap<>(original.size());
for(HashMap.Entry<String,List<Integer>> entry : original) {
    for(Integer n : entry.getValue()) {
        if(!reverse.containsKey(n)) { //Reverse doesn't have this number yet, create a new list at that key.
            reverse.add(n,new ArrayList<>());
        }
        reverse.get(n).add(entry.getKey());
    }
}

让我知道其中是否有错误，但请先修补它！