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How to implement text Auto-complete feature using Ternary Search Tree
• Last Updated : 26 Jun, 2020

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.

## Recommended: Please try your approach on {IDE} first, before moving on to the solution.

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 ``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& 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 util(Node* root,``                    ``string pattern)``{``    ``// Stores the words``    ``// to suggest``    ``char` `buffer;`` ` `    ``vector 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 autocomplete(Node* root,``                            ``string pattern)``{``    ``vector 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``            ``root = root->eq;`` ` `        ``// If not found``        ``else``            ``return` `words;`` ` `        ``pos++;``    ``}`` ` `    ``// Search for all the words``    ``// from the current node``    ``words = util(root, pattern);`` ` `    ``return` `words;``}`` ` `// Function to print``// suggested words`` ` `void` `print(vector sugg,``           ``string pat)``{``    ``for` `(``int` `i = 0; i < sugg.size();``         ``i++)``        ``cout << pat << sugg[i].c_str()``             ``<< ``"\n"``;``}`` ` `// Driver Code``int` `main()``{``    ``vector 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 sugg``        ``= autocomplete(tree, pat);`` ` `    ``if` `(sugg.size() == 0)``        ``cout << ``"None"``;`` ` `    ``else``        ``print(sugg, pat);`` ` `    ``return` `0;``}`
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.

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