Auto-complete feature using Trie


We are given a Trie with a set of strings stored in it. Now the user types in a prefix of his search query, we need to give him all recommendations to auto-complete his query based on the strings stored in the Trie. We assume that the Trie stores past searches by the users.

For example if the Trie store {“abc”, “abcd”, “aa”, “abbbaba”} and the User types in “ab” then he must be shown {“abc”, “abcd”, “abbbaba”}.

Prerequisite Trie Search and Insert.

Given a query prefix, we search for all words having this query.

  1. Search for given query using standard Trie search algorithm.
  2. If query prefix itself is not present, return -1 to indicate the same.
  3. If query is present and is end of word in Trie, print query. This can quickly checked by seeing if last matching node has isEndWord flag set. We use this flag in Trie to mark end of word nodes for purpose of searching.
  4. If last matching node of query has no children, return.
  5. Else recursively print all nodes under subtree of last matching node.

Below is C++ implementation of above steps.

// C++ program to demonstrate auto-complete feature
// using Trie data structure.
using namespace std;

// Alphabet size (# of symbols)
#define ALPHABET_SIZE (26)

// Converts key current character into index
// use only 'a' through 'z' and lower case
#define CHAR_TO_INDEX(c) ((int)c - (int)'a')

// trie node
struct TrieNode
    struct TrieNode *children[ALPHABET_SIZE];

    // isWordEnd is true if the node represents
    // end of a word
    bool isWordEnd;

// Returns new trie node (initialized to NULLs)
struct TrieNode *getNode(void)
    struct TrieNode *pNode = new TrieNode;
    pNode->isWordEnd = false;

    for (int i = 0; i < ALPHABET_SIZE; i++)
        pNode->children[i] = NULL;

    return pNode;

// If not present, inserts key into trie.  If the
// key is prefix of trie node, just marks leaf node
void insert(struct TrieNode *root,  const string key)
    struct TrieNode *pCrawl = root;

    for (int level = 0; level < key.length(); level++)
        int index = CHAR_TO_INDEX(key[level]);
        if (!pCrawl->children[index])
            pCrawl->children[index] = getNode();

        pCrawl = pCrawl->children[index];

    // mark last node as leaf
    pCrawl->isWordEnd = true;

// Returns true if key presents in trie, else false
bool search(struct TrieNode *root, const string key)
    int length = key.length();
    struct TrieNode *pCrawl = root;
    for (int level = 0; level < length; level++)
        int index = CHAR_TO_INDEX(key[level]);

        if (!pCrawl->children[index])
            return false;

        pCrawl = pCrawl->children[index];

    return (pCrawl != NULL && pCrawl->isWordEnd);

// Returns 0 if current node has a child
// If all children are NULL, return 1.
bool isLastNode(struct TrieNode* root)
    for (int i = 0; i < ALPHABET_SIZE; i++)
        if (root->children[i])
            return 0;
    return 1;

// Recursive function to print auto-suggestions for given
// node.
void suggestionsRec(struct TrieNode* root, string currPrefix)
    // found a string in Trie with the given prefix
    if (root->isWordEnd)
        cout << currPrefix;
        cout << endl;

    // All children struct node pointers are NULL
    if (isLastNode(root))

    for (int i = 0; i < ALPHABET_SIZE; i++)
        if (root->children[i])
            // append current character to currPrefix string
            currPrefix.push_back(97 + i);

            // recur over the rest
            suggestionsRec(root->children[i], currPrefix);

// print suggestions for given query prefix.
int printAutoSuggestions(TrieNode* root, const string query)
    struct TrieNode* pCrawl = root;

    // Check if prefix is present and find the
    // the node (of last level) with last character
    // of given string.
    int level;
    int n = query.length();
    for (level = 0; level < n; level++)
        int index = CHAR_TO_INDEX(query[level]);

        // no string in the Trie has this prefix
        if (!pCrawl->children[index])
            return 0;

        pCrawl = pCrawl->children[index];

    // If prefix is present as a word.
    bool isWord = (pCrawl->isWordEnd == true);

    // If prefix is last node of tree (has no
    // children)
    bool isLast = isLastNode(pCrawl);

    // If prefix is present as a word, but
    // there is no subtree below the last
    // matching node.
    if (isWord && isLast)
        cout << query << endl;
        return -1;

    // If there are are nodes below last
    // matching character.
    if (!isLast)
        string prefix = query;
        suggestionsRec(pCrawl, prefix);
        return 1;

// Driver Code
int main()
    struct TrieNode* root = getNode();
    insert(root, "hello");
    insert(root, "dog");
    insert(root, "hell");
    insert(root, "cat");
    insert(root, "a");
    insert(root, "hel");
    insert(root, "help");
    insert(root, "helps");
    insert(root, "helping");
    int comp = printAutoSuggestions(root, "hel");

    if (comp == -1)
        cout << "No other strings found with this prefix\n";

    else if (comp == 0)
        cout << "No string found with this prefix\n";

    return 0;



How can we improve this?
The number of matches might just be too large so we have to be selective while displaying them. We can restrict ourselves to display only the relevant results. By relevant, we can consider the past search history and show only the most searched matching strings as relevant results.
Store another value for the each node where isleaf=True which contains the number of hits for that query search. For example if “hat” is searched 10 times, then we store this 10 at the last node for “hat”. Now when we want to show the recommendations, we display the top k matches with the highest hits. Try to implement this on your own.

This article is contributed by Hemang Sarkar. If you like GeeksforGeeks and would like to contribute, you can also write an article using or mail your article to See your article appearing on the GeeksforGeeks main page and help other Geeks.

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