Update all contacts phone number with associated account's phone number when associated account is updated

Assume Phone field of Account and Phone field of contact are used here in example.
Sample Code:
trigger updatePhone on Account (after update) {
    Set <Id> accountIdSet = new Set <Id>();
    Map <Id,String> accountMap = new Map <Id,string> ();
    for(Account acc:trigger.new) {
        if(Trigger.oldMap.get(acc.Id).phone != acc.phone) {
            accountIdSet.add(acc.id);
        }
        if(acc.phone != null) {
            accountMap.put(acc.id,acc.phone);
        }
    }
    List <contact> contactList = new List <Contact>();
    for(Contact con:[select id,accountId,Phone from contact where accountId in:accountIdSet]) {
        if(accountMap != null && accountMap.containsKey(con.accountId)) {
            con.phone = accountMap.get(con.accountId);
            contactList.add(con);
        }
    }
    if(contactList.size() >0) {
        update contactList;
    }
}
Output: