How to save map with List as child and Id as parent

In this following Example,I had taken case as a example

Map<Id, List<Case>> parentChildCase = new Map<Id, List<Case>>();

Map<ID, Case> childMap = new Map<ID, Case>([SELECT Id, ..., FROM Case]);

for(ID c : childMap .keySet())

{

    if(parentChildCase.containsKey(childMap .get(c).ID))

    {

        parentChildCase.get(cMap.get(c).ID).add(childMap .get(c));

    }

    else

    {

        List<Case> cases = new List<Case>();

        cases.add(childMap .get(c));

        parentChildCase.put(childMap .get(c).ID, cases);

    }

}