Hi Murali Krishna,

I am sharing with you the apex class code for getting the Map of accountId Vs List<Case> Map 

public class AccountCaseController{
	public static Map<Id, List<Case>>  getMapOfCase(){
	
		List<Case> caseList =[SELECT Id, ContactId FROM Case];
		Set<Id> conIds = new Set<Id>();
		Map<Id, List<Case>> conIdVsCaseMap = new Map<Id, List<Case>>();
		for(Case c : caseList){
		 conIds.add(c.ContactId);
		 
		 if(!conIdVsCaseMap.containskey(c.ContactId) ){
			conIdVsCaseMap.put(c.ContactId, new List<Case>());
		 }
		 conIdVsCaseMap.get(c.ContactId).add(c);
		}
		
		
		List<Contact> conlist =[SELECT Id,AccountId FROM Contact WHERE Id IN : conIds];
		Set<Id> accIds = new Set<Id>();
		Map<Id, List<Contact>> accIdVsConMap = new Map<Id, List<Contact>>();
		for(Contact con : conlist){
		 accIds.add(con.AccountId);
		 
		 if(!accIdVsConMap.containskey(con.AccountId) ){
			accIdVsConMap.put(con.AccountId, new List<Case>());
		 }
		 accIdVsConMap.get(con.AccountId).add(con);
		}
		
		
		Map<Id, List<Case>> accIdVsCaseMap = new Map<Id, List<Case>>();
		for(Id accId : accIdVsConMap.keyset()){
		 for(Contact con : accIdVsConMap.get(accId)){
		 
		 if(!accIdVsCaseMap.containskey(accId) ){
			accIdVsConMap.put(accId, new List<Case>());
		 }
		 accIdVsConMap.get(accId).addAll(conIdVsCaseMap.get(con.Id));
		}
		
		return accIdVsConMap;
	}
}

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Thanks