Maps-PhoneToAccount.cls

/*
Implement the method phoneToAccount that takes as input a list of Accounts, and returns a Map containing the phone number as a key and the Account record as the value. Do not include accounts without a phone number.

For example, given the following accounts:

a1
Name: 'Acme Enterprises'
Phone: '2143452398'
a2
Name: 'Universal Containers'
Phone: '6923849837'
The method call phoneToAccount(new List {a1, a2}) should return a map containing the following key/value pairs:

'2143452398' -> a1
'6923849837' -> a2
*/
public Map<String, Account> phoneToAccount(List<Account> accounts) {
    //code here
    Map<String,Account> book=new Map<String,Account>();
    for(Account a: accounts){
        if(a.Phone!=null){
            book.put(a.Phone,a);
        }
    }
    return book;
}