CareerCup

what do you mean by "true collar"?

True Collar... ROFL.. I think he meant True Caller..

Well you can use Map.
Map<String, String> map = new HashMap<String, String>();

Which maps, Any number (converted to String) to username in String

In such scenarios, TRIE is the best data structure for storing the contacts.

class Country
{
String Name;
int countrycode; // to search by country
}
class RegionalCode
{
Country c;
Hashmap<RegionalCode,Trie>;
int regionalcode;
String RegionName;
}
class Trie
{
int number;
LinkedList<Integer>child;
}

So, let's start by first analyzing how Truecaller works

Case 1 : If a user has truecaller installed in his phone and he sets his profile name then his number is directly mapped to his username and that will be displayed whenever he calls someone who has truecaller installed

Case 2 : This is the interesting case as this shows the true utility of Truecaller. Truecaller also had access to contacts and so uploads all the contacts saved by people to their servers. Now, they calculate the name for a number by calculating the highest frequency of exact or matching words used for a particular phone number.

So, we need a centralized service that syncs contacts for each user. Then, a periodic backend job that keeps updating the internal directory or map of contacts for our Truecaller or whatever we choose to name our app.

Press editor

map<string,string>Map;
    Map[ "true" ]="collar";