如何反转HashMap的键和值？

Question

我已创建HashMap<String,List<Integer>>。现在我想通过替换第一张地图上的键和值来创建反向HashMap<Integer,List<String>>。

例如

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

反转HashMap中：{ 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

尝试此。

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()); 
    } 
} 

让我知道是否有一些错误，但先修补它！

Answer 3

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]}