Map<Id, List<Contact>> accountToContacts = new Map<Id, List<Contact>>();
// ... map population logic ...
Id accId1 = '001000000000001AAA';
List<Contact> list1 = new List<Contact>();
list1.add(new Contact(LastName = 'Smith'));
list1.add(new Contact(LastName = 'Jones'));
accountToContacts.put(accId1, list1);
Id accId2 = '001000000000002BBB';
List<Contact> list2 = new List<Contact>();
list2.add(new Contact(LastName = 'Brown'));
accountToContacts.put(accId2, list2);
for (Id accId : accountToContacts.keySet()) {
    // Inner loop to iterate over the List value
    for (Contact con : accountToContacts.get(accId)) {
        System.debug('Account ID: ' + accId + ' | Contact: ' + con.LastName);
    }
}
 

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