## Monday, October 10, 2016

### Implement a Phone Directory - GeeksforGeeks

Implement a Phone Directory - GeeksforGeeks
Given a list of contacts which exist in a phone directory. The task is to implement search query for the phone directory. The search query on a string 'str' displays all the contacts which prefix 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 lower case alphabets.
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

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

class TrieNode
{
// Each Trie Node contains a Map 'child'
// where each alphabet points to a Trie
// Node.
HashMap<Character,TrieNode> child;

// 'isLast' is true if the node represents
// end of a contact
boolean isLast;

// Default Constructor
public TrieNode()
{
child = new HashMap<Character,TrieNode>();

// Initialize all the Trie nodes with NULL
for (char i = 'a'; i <= 'z'; i++)
child.put(i,null);

isLast = false;
}
}

class Trie
{
TrieNode root;

// Insert all the Contacts into the Trie
public void insertIntoTrie(String contacts[])
{
root = new TrieNode();
int n = contacts.length;
for (int i = 0; i < n; i++)
{
insert(contacts[i]);
}
}

// Insert a Contact into the Trie
public void insert(String s)
{
int len = s.length();

// 'itr' is used to iterate the Trie Nodes
TrieNode itr = root;
for (int i = 0; i < len; i++)
{
// Check if the s[i] is already present in
// Trie
TrieNode nextNode = itr.child.get(s.charAt(i));
if (nextNode == null)
{
nextNode = new TrieNode();

// Insert into the HashMap
itr.child.put(s.charAt(i),nextNode);
}

// Move the iterator('itr') ,to point to next
// Trie Node
itr = nextNode;

// If its the last character of the string 's'
// then mark 'isLast' as true
if (i == len - 1)
itr.isLast = true;
}
}

// This function simply displays all dictionary words
// going through current node.  String 'prefix'
// represents string corresponding to the path from
// root to curNode.
public void displayContactsUtil(TrieNode curNode,
String prefix)
{

// Check if the string 'prefix' ends at this Node
// If yes then display the string found so far
if (curNode.isLast)
System.out.println(prefix);

// Find all the adjacent Nodes to the current
// Node and then call the function recursively
// This is similar to performing DFS on a graph
for (char i = 'a'; i <= 'z'; i++)
{
TrieNode nextNode = curNode.child.get(i);
if (nextNode != null)
{
displayContactsUtil(nextNode, prefix + i);
}
}
}

// Display suggestions after every character enter by
// the user for a given string 'str'
void displayContacts(String str)
{
TrieNode prevNode = root;

// 'flag' denotes whether the string entered
// so far is present in the Contact List

String prefix = "";
int len = str.length();

// Display the contact List for string formed
// after entering every character
int i;
for (i = 0; i < len; i++)
{
// 'str' stores the string entered so far
prefix += str.charAt(i);

// Get the last character entered
char lastChar = prefix.charAt(i);

// Find the Node corresponding to the last
// character of 'str' which is pointed by
// prevNode of the Trie
TrieNode curNode = prevNode.child.get(lastChar);

// If nothing found, then break the loop as
// no more prefixes are going to be present.
if (curNode == null)
{
+ prefix + "\"");
i++;
break;
}

// If present in trie then display all
// the contacts with given prefix.
System.out.println("\nSuggestions based on \""
+ prefix + "\" are");
displayContactsUtil(curNode, prefix);

// Change prevNode for next prefix
prevNode = curNode;
}

for ( ; i < len; i++)
{
prefix += str.charAt(i);
+ prefix + "\"");
}
}
}