Implement a Phone Directory

Given a list of contacts that exist in a phone directory. The task is to implement a search query for the phone directory. The search query on a string ‘str’ displays all the contacts which prefixes as ‘str’. One special property of the search function is that when a user searches for a contact from the contact list then suggestions (Contacts with prefix as the string entered so for) are shown after user enters each character.
Note: Contacts in the list consist of only lowercase alphabets. Example:

Input : contacts [] = {“gforgeeks” , “geeksquiz” }
 Query String = “gekk”

Output : Suggestions based on "g" are 
 geeksquiz
 gforgeeks

 Suggestions based on "ge" are 
 geeksquiz

 No Results Found for "gek" 

 No Results Found for "gekk" 

Phone Directory can be efficiently implemented using Trie Data Structure. We insert all the contacts into Trie. Generally search query on a Trie is to determine whether the string is present or not in the trie, but in this case we are asked to find all the strings with each prefix of ‘str’. This is equivalent to doing a DFS traversal on a graph. From a Trie node, visit adjacent Trie nodes and do this recursively until there are no more adjacent. This recursive function will take 2 arguments one as Trie Node which points to the current Trie Node being visited and other as the string which stores the string found so far with prefix as ‘str’. Each Trie Node stores a boolean variable ‘isLast’ which is true if the node represents end of a contact(word).

// This function displays all words with given
// prefix. "node" represents last node when 
// path from root follows characters of "prefix".
displayContacts (TreiNode node, string prefix)
 If (node.isLast is true)
 display prefix

 // finding adjacent nodes
 for each character ‘i’ in lower case Alphabets 
 if (node.child[i] != NULL)
 displayContacts(node.child[i], prefix+i)

User will enter the string character by character and we need to display suggestions with the prefix formed after every entered character. So one approach to find the prefix starting with the string formed is to check if the prefix exists in the Trie, if yes then call the displayContacts() function. In this approach after every entered character we check if the string exists in the Trie. Instead of checking again and again, we can maintain a pointer prevNode' that points to the TrieNode which corresponds to the last entered character by the user, now we need to check the child node for the 'prevNode' when user enters another character to check if it exists in the Trie. If the new prefix is not in the Trie, then all the string which are formed by entering characters after 'prefix' can't be found in Trie too. So we break the loop that is being used to generate prefixes one by one and print "No Result Found" for all remaining characters. 

Suggestions based on gare geeksquiz
gforgeeks

Suggestions based on geare geeksquiz

No Results Found for gek

No Results Found for gekk

Time Complexity: O(n*m) where n is the number of contacts and m is the maximum length of a contact string.
Auxiliary Space: O(n*m)