How to implement text Auto-complete feature using Ternary Search Tree

Given a set of strings S and a string patt the task is to autocomplete the string patt to strings from S that have patt as a prefix, using a Ternary Search Tree. If no string matches the given prefix, print “None”.
Examples:

Input: S = {“wallstreet”, “geeksforgeeks”, “wallmart”, “walmart”, “waldomort”, “word”], 
patt = “wall” 
Output:
wallstreet 
wallmart 
Explanation:
Only two strings {“wallstreet”, “wallmart”} from S matches with the given pattern “wall”.
Input: S = {“word”, “wallstreet”, “wall”, “wallmart”, “walmart”, “waldo”, “won”}, patt = “geeks” 
Output: None 
Explanation:
Since none of word of set matches with pattern so empty list will be printed. 

Trie Approach: Please refer this article to learn about the implementation using Trie data structure.
Ternary Search Tree Approach Follow the steps below to solve the problem: 

• Insert all the characters of the strings in S into the Ternary Search Tree based on the following conditions: 
  1. If the character to be inserted is smaller than current node value, traverse the left subtree.
  2. If the character to be inserted is greater than current node value, traverse the right subtree to.
  3. If the character to be inserted is same as that of the current node value, traverse the equal subtree if it is not the end of the word. If so, mark the node as the end of the word.
• Follow a similar approach for extracting suggestions.
• Traverse the tree to search the given prefix patt following a similar traversal technique as mentioned above.
• If the given prefix is not found, print “None”.
• If the given prefix is found, traverse the tree from the node where the prefix ends. Traverse the left subtree and generate suggestions followed by the right and equal subtrees from every node .
• Every time a node is encountered which has the endofWord variable set, it denotes that a suggestion has been obtained. Insert that suggestion into words.
• Return words after generating all possible suggestions.

Below is the implementation of the above approach: 

C++

// C++ Program to generate
// autocompleted texts from
// a given prefix using a
// Ternary Search Tree
#include <bits/stdc++.h>
using namespace std;

// Define the Node of the
// tree
struct Node {

    // Store the character
    // of a string
    char data;
    // Store the end of
    // word
    int end;
    // Left Subtree
    struct Node* left;

    // Equal Subtree
    struct Node* eq;

    // Right Subtree
    struct Node* right;
};

// Function to create a Node
Node* createNode(char newData)
{
    struct Node* newNode = new Node();
    newNode->data = newData;
    newNode->end = 0;
    newNode->left = NULL;
    newNode->eq = NULL;
    newNode->right = NULL;
    return newNode;
}

// Function to insert a word
// in the tree
void insert(Node** root,
            string word,
            int pos = 0)
{

    // Base case
    if (!(*root))
        *root = createNode(word[pos]);

    // If the current character is
    // less than root's data, then
    // it is inserted in the
    // left subtree

    if ((*root)->data > word[pos])
        insert(&((*root)->left), word,
            pos);

    // If current character is
    // more than root's data, then
    // it is inserted in the right
    // subtree

    else if ((*root)->data < word[pos])
        insert(&((*root)->right), word,
            pos);

    // If current character is same
    // as that of the root's data

    else {
        // If it is the end of word

        if (pos + 1 == word.size())
            // Mark it as the
            // end of word
            (*root)->end = 1;

        // If it is not the end of
        // the string, then the
        // current character is
        // inserted in the equal subtree

        else
            insert(&((*root)->eq), word, pos + 1);
    }
}

// Function to traverse the ternary search tree
void traverse(Node* root,
            vector<string>& ret,
            char* buff,
            int depth = 0)
{
    // Base case
    if (!root)
        return;
    // The left subtree is
    // traversed first
    traverse(root->left, ret,
            buff, depth);

    // Store the current character
    buff[depth] = root->data;

    // If the end of the string
    // is detected, store it in
    // the final ans
    if (root->end) {
        buff[depth + 1] = '\0';
        ret.push_back(string(buff));
    }

    // Traverse the equal subtree
    traverse(root->eq, ret,
            buff, depth + 1);

    // Traverse the right subtree
    traverse(root->right, ret,
            buff, depth);
}

// Utility function to find
// all the words
vector<string> util(Node* root,
                    string pattern)
{
    // Stores the words
    // to suggest
    char buffer[1001];

    vector<string> ret;

    traverse(root, ret, buffer);

    if (root->end == 1)
        ret.push_back(pattern);
    return ret;
}

// Function to autocomplete
// based on the given prefix
// and return the suggestions
vector<string> autocomplete(Node* root,
                            string pattern)
{
    vector<string> words;
    int pos = 0;

    // If pattern is empty
    // return an empty list
    if (pattern.empty())
        return words;

    // Iterating over the characters
    // of the pattern and find it's
    // corresponding node in the tree

    while (root && pos < pattern.length()) {

        // If current character is smaller
        if (root->data > pattern[pos])
            // Search the left subtree
            root = root->left;

        // current character is greater
        else if (root->data < pattern[pos])
            // Search right subtree
            root = root->right;

        // If current character is equal
        else if (root->data == pattern[pos]) {

            // Search equal subtree
            // since character is found, move to the next character in the pattern
            root = root->eq;
            pos++;
        }

        // If not found
        else
            return words;

    }

    // Search for all the words
    // from the current node
    words = util(root, pattern);

    return words;
}

// Function to print
// suggested words

void print(vector<string> sugg,
        string pat)
{
    for (int i = 0; i < sugg.size();
        i++)
        cout << pat << sugg[i].c_str()
            << "\n";
}

// Driver Code
int main()
{
    vector<string> S
        = { "wallstreet", "geeksforgeeks",
            "wallmart", "walmart",
            "waldormort", "word" };

    Node* tree = NULL;

    // Insert the words in the
    // Ternary Search Tree
    for (string str : S)
        insert(&tree, str);

    string pat = "wall";

    vector<string> sugg
        = autocomplete(tree, pat);

    if (sugg.size() == 0)
        cout << "None";

    else
        print(sugg, pat);

    return 0;
}

Python3

# Python Program to generate
# autocompleted texts from
# a given prefix using a
# Ternary Search Tree

# Define the Node of the
# tree
class Node:

    # Store the character
    # of a string
    def __init__(self, newData):
        self.data = newData
        # Store the end of
        # word
        self.end = 0
        # Left Subtree
        self.left = None

        # Equal Subtree
        self.eq = None

        # Right Subtree
        self.right = None

# Function to create a Node
# Function to create a Node
def createNode(newData):
    newNode = Node(newData)
    newNode.end = 0
    newNode.left = None
    newNode.eq = None
    newNode.right = None
    return newNode


# Function to insert a word
# in the tree
def insert(root, word, pos = 0):

    # Base case
    if not root:
        root = createNode(word[pos])

    # If the current character is
    # less than root's data, then
    # it is inserted in the
    # left subtree
    if root.data > word[pos]:
        root.left = insert(root.left, word, pos)

    # If current character is
    # more than root's data, then
    # it is inserted in the right
    # subtree
    elif root.data < word[pos]:
        root.right = insert(root.right, word, pos)

    # If current character is same
    # as that of the root's data
    else:
        # If it is the end of word
        if pos + 1 == len(word):
            # Mark it as the
            # end of word
            root.end = 1
        # If it is not the end of
        # the string, then the
        # current character is
        # inserted in the equal subtree
        else:
            root.eq = insert(root.eq, word, pos + 1)
    return root



# Function to traverse the ternary search tree
def traverse(root, ret, buff, depth = 0):

    # Base case
    if not root:
        return

    # The left subtree is
    # traversed first
    traverse(root.left, ret, buff, depth)

    # Store the current character
    buff[depth] = root.data

    # If the end of the string
    # is detected, store it in
    # the final ans
    if root.end:
        buff[depth + 1] = '\0'
        ret.append("".join(buff[:depth + 1]))

    # Traverse the equal subtree
    traverse(root.eq, ret, buff, depth + 1)

    # Traverse the right subtree
    traverse(root.right, ret, buff, depth)

# Utility function to find
# all the words
def util(root, pattern):

    # Stores the words
    # to suggest
    buffer = [None] * 1001

    ret = []

    traverse(root, ret, buffer)

    if root.end == 1:
        ret.append(pattern)
    return ret

# Function to autocomplete
# based on the given prefix
# and return the suggestions
def autocomplete(root, pattern):
    words = []
    pos = 0

    # If pattern is empty
    # return an empty list
    if not pattern:
        return words

    # Iterating over the characters
    # of the pattern and find it's
    # corresponding node in the tree
    while root and pos < len(pattern):
        # If current character is smaller
        if root.data > pattern[pos]:
            # Search the left subtree
            root = root.left

        # current character is greater
        elif root.data < pattern[pos]:
            # Search right subtree
            root = root.right

        # If current character is equal
        elif root.data == pattern[pos]:
            # Search equal subtree
            # since character is found, move to the next character in the pattern
            root = root.eq
            pos += 1

        # If not found
        else:
            return words

    # Search for all the words
    # from the current node
    words = util(root, pattern)

    return words

# Function to print
# suggested words
def print_suggestions(sugg, pat):
    for sug in sugg:
        print(pat + sug)

# Driver Code
if __name__ == '__main__':
    S = ['wallstreet', 'geeksforgeeks', 'wallmart', 'walmart', 'waldormort', 'word']
    tree = None

    # Insert the words in the
    # Ternary Search Tree
    for str in S:
        tree = insert(tree, str)

    pat = 'wall'
    sugg = autocomplete(tree, pat)

    if not sugg:
        print('None')
    else:
        print_suggestions(sugg, pat)

# This code is contributed by Utkarsh Kumar

C#

// C# Program to generate
// autocompleted texts from
// a given prefix using a
// Ternary Search Tree

using System;
using System.Collections.Generic;

namespace TernarySearchTree {
public class TernarySearchTree {
    // Define the Node of the
    // tree
    private class Node {
        public char Value
        {
            get;
            set;
        }
        // Store the end of
        // word
        public bool IsEndOfWord
        {
            get;
            set;
        }
        // Left Subtree
        public Node Left
        {
            get;
            set;
        }
        // Equal Subtree
        public Node Middle
        {
            get;
            set;
        }
        // Right Subtree
        public Node Right
        {
            get;
            set;
        }
    }

    private Node _root;

    public void Insert(string word)
    {
        _root = Insert(_root, word, 0);
    }

    // Function to insert a word
    // in the tree
    private Node Insert(Node node, string word, int index)
    {
        // Base case
        if (node == null) {
            node = new Node{ Value = word[index] };
        }
        // If the current character is
        // less than root's data, then
        // it is inserted in the
        // left subtree
        if (word[index] < node.Value) {
            node.Left = Insert(node.Left, word, index);
        }
        // If current character is
        // more than root's data, then
        // it is inserted in the right
        // subtree
        else if (word[index] > node.Value) {
            node.Right = Insert(node.Right, word, index);
        }
        else {
            if (index < word.Length - 1) {
                node.Middle
                    = Insert(node.Middle, word, index + 1);
            }
            else {
                node.IsEndOfWord = true;
            }
        }

        return node;
    }

    public List<string> AutoComplete(string prefix)
    {
        var results = new List<string>();
        var node = FindNode(prefix);
        // Base case
        if (node != null) {
            if (node.IsEndOfWord) {
                results.Add(prefix);
            }

            Traverse(node.Middle, prefix, results);
        }

        return results;
    }

    private Node FindNode(string prefix)
    {
        var node = _root;
        var index = 0;

        while (node != null && index < prefix.Length) {
            if (prefix[index] < node.Value) {
                node = node.Left;
            }
            else if (prefix[index] > node.Value) {
                node = node.Right;
            }
            else {
                index++;

                if (index < prefix.Length) {
                    node = node.Middle;
                }
            }
        }

        return node;
    }
    // Traversing
    private void Traverse(Node node, string prefix,
                          List<string> results)
    {
        if (node != null) {
            Traverse(node.Left, prefix, results);

            var newPrefix = prefix + node.Value;

            if (node.IsEndOfWord) {
                results.Add(newPrefix);
            }

            Traverse(node.Middle, newPrefix, results);
            Traverse(node.Right, prefix, results);
        }
    }
}
// Driver Code
class Program {
    static void Main(string[] args)
    {
        // Insert the words in the
        // Ternary Search Tree
        var t = new TernarySearchTree();
        t.Insert("wallstreet");
        t.Insert("geeksforgeeks");
        t.Insert("wallmart");
        t.Insert("walmart");
        t.Insert("waldormort");
        t.Insert("word");

        var results = t.AutoComplete("wall");
        foreach(var r in results) { Console.WriteLine(r); }
    }
}
}
// Code is contributed by Narasinga Nikhil

Javascript

// JavaScript Program for the above approach

// Define the Node of the tree
class Node
{

  // Store the character of a string
  constructor(newData)
  {
    this.data = newData;

    // Store the end of word
    this.end = 0;

    // Left Subtree
    this.left = null;

    // Equal Subtree
    this.eq = null;

    // Right Subtree
    this.right = null;
  }
}

// Function to create a Node
function createNode(newData) {
  const newNode = new Node(newData);
  newNode.end = 0;
  newNode.left = null;
  newNode.eq = null;
  newNode.right = null;
  return newNode;
}

// Function to insert a word in the tree
function insert(root, word, pos = 0)
{

  // Base case
  if (!root) {
    root = createNode(word[pos]);
  }

  // If the current character is less than root's data,
  // then it is inserted in the left subtree
  if (root.data > word[pos]) {
    root.left = insert(root.left, word, pos);
  }

  // If current character is more than root's data,
  // then it is inserted in the right subtree
  else if (root.data < word[pos]) {
    root.right = insert(root.right, word, pos);
  }

  // If current character is same as that of the root's data
  else
  {

    // If it is the end of word
    if (pos + 1 === word.length)
    {

      // Mark it as the end of word
      root.end = 1;
    }

    // If it is not the end of the string, then the
    // current character is inserted in the equal subtree
    else {
      root.eq = insert(root.eq, word, pos + 1);
    }
  }
  return root;
}

// Function to traverse the ternary search tree
function traverse(root, ret, buff, depth = 0) {
  // Base case
  if (!root) {
    return;
  }

  // The left subtree is traversed first
  traverse(root.left, ret, buff, depth);

  // Store the current character
  buff[depth] = root.data;

  // If the end of the string is detected, store it in the final ans
  if (root.end) {
    buff[depth + 1] = '\0';
    ret.push(buff.slice(0, depth + 1).join(""));
  }

  // Traverse the equal subtree
  traverse(root.eq, ret, buff, depth + 1);

  // Traverse the right subtree
  traverse(root.right, ret, buff, depth);
}

// Utility function to find all the words
function util(root, pattern)
{

  // Stores the words to suggest
  const buffer = Array.from({ length: 1001 }, () => null);
  const ret = [];
  traverse(root, ret, buffer);
  if (root.end === 1) {
    ret.push(pattern);
  }
  return ret;
}

// Function to autocomplete
// based on the given prefix
// and return the suggestions
function autocomplete(root, pattern) {
  let words = [];
  let pos = 0;

  // If pattern is empty
  // return an empty list
  if (!pattern) {
    return words;
  }

  // Iterating over the characters
  // of the pattern and find it's
  // corresponding node in the tree
  while (root && pos < pattern.length)
  {

    // If current character is smaller
    if (root.data > pattern[pos])
    {

      // Search the left subtree
      root = root.left;
    }

    // current character is greater
    else if (root.data < pattern[pos])
    {

      // Search right subtree
      root = root.right;
    }

    // If current character is equal
    else if (root.data === pattern[pos])
    {

      // Search equal subtree
      // since character is found, move to
      // the next character in the pattern
      root = root.eq;
      pos += 1;
    }
    // If not found
    else {
      return words;
    }
  }

  // Search for all the words
  // from the current node
  words = util(root, pattern);

  return words;
}

// Function to print
// suggested words
function printSuggestions(sugg, pat) {
    for (let sug of sugg) {
        console.log(pat + sug);
    }
}

// Driver Code
let S = ['wallstreet', 'geeksforgeeks', 'wallmart', 'walmart', 'waldormort', 'word'];
let tree = null;

// Insert the words in the
// Ternary Search Tree
for (let str of S) {
    tree = insert(tree, str);
}

let pat = 'wall';
let sugg = autocomplete(tree, pat);

if (!sugg) {
    console.log('None');
} else {
    printSuggestions(sugg, pat);
}

// This code is contributed by codebraxnzt

Java

// java Program to generate
// autocompleted texts from
// a given prefix using a
// Ternary Search Tree

// The TSTNode class defines the nodes of the tree.
class TSTNode {
    char data;
    boolean isEndOfWord;
    TSTNode left, middle, right;

    public TSTNode(char data)
    {
        this.data = data;
        this.isEndOfWord = false;
        this.left = null;
        this.middle = null;
        this.right = null;
    }
}

// The TernarySearchTree class defines the tree itself
class TernarySearchTree {
    TSTNode root;

    public TernarySearchTree() { this.root = null; }

    public void insert(String word)
    {
        root = insert(root, word.toCharArray(), 0);
    }

    // The insert() method inserts a given string into the
    // tree recursively.
    private TSTNode insert(TSTNode node, char[] word,
                           int index)
    {
        if (node == null) {
            node = new TSTNode(word[index]);
        }

        if (word[index] < node.data) {
            node.left = insert(node.left, word, index);
        }
        else if (word[index] > node.data) {
            node.right = insert(node.right, word, index);
        }
        else {
            if (index + 1 < word.length) {
                node.middle
                    = insert(node.middle, word, index + 1);
            }
            else {
                node.isEndOfWord = true;
            }
        }
        return node;
    }

    public void autoComplete(String prefix)
    {
        TSTNode node = searchPrefix(prefix);
        if (node == null) {
            System.out.println(
                "No words found with given prefix");
            return;
        }

        if (node.isEndOfWord) {
            System.out.println(prefix);
        }

        traverse(node.middle, new StringBuilder(prefix));
    }
    // The searchPrefix() method searches for a given prefix
    // in the tree.
    private TSTNode searchPrefix(String prefix)
    {
        TSTNode node = root;
        int i = 0;
        while (node != null && i < prefix.length()) {
            if (prefix.charAt(i) < node.data) {
                node = node.left;
            }
            else if (prefix.charAt(i) > node.data) {
                node = node.right;
            }
            else {
                i++;
                if (i == prefix.length()) {
                    return node;
                }
                node = node.middle;
            }
        }
        return node;
    }

    private void traverse(TSTNode node,
                          StringBuilder prefix)
    {
        if (node == null) {
            return;
        }

        traverse(node.left, prefix);

        if (node.isEndOfWord) {
            System.out.println(prefix.toString()
                               + node.data);
        }

        traverse(node.middle, prefix.append(node.data));
        prefix.deleteCharAt(prefix.length() - 1);

        traverse(node.right, prefix);
    }
}
// Driver code
public class Main {
    public static void main(String[] args)
    {
        String[] words
            = { "wallstreet", "geeksforgeeks", "wallmart",
                "walmart",    "waldomort",     "word" };
        String prefix = "wall";

        TernarySearchTree tst = new TernarySearchTree();
        for (String word : words) {
            tst.insert(word);
        }

        System.out.println("Words with prefix " + prefix
                           + " are:");
        tst.autoComplete(prefix);
    }
}

Output

wallmart
wallstreet

Time Complexity: O(L* log N) where L is length of longest word. 
The space is proportional to the length of the string to be stored.
Auxiliary Space: O(N)