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Count the number of words with given prefix using Trie
• Difficulty Level : Easy
• Last Updated : 17 Dec, 2020

Prerequisite: Trie

Given a list of string str[] and a prefix string pre. The task is to count the number of words in list of string with given prefix using trie.

Examples:

Input: str = [ “apk”, “app”, “apple”, “arp”, “array” ], pre = “ap”
Output:
Explanation:
Below is the representation of trie from using above string. The words in str having prefix “ap” are apk, app and apple
So, the count is 3

Input: str = [ “gee”, “geek”, “geezer”, “geeksforgeeks”, “geekiness”, “geekgod” ], pre = “geek”
Output:

Approach:
To solve this problem Trie Data Structure is used and each node of this Trie contains the following three fields:

1. children: This field is used for mapping from a character to the next level trie node
2. isEndOfWord: This field is used to distinguish the node as end of word node
3. num: This field is used to count the number of times a node is visited during insertion in trie

Steps:

• Insert list of string in trie such that every string in the list is inserted as an individual trie node.
• During inserting update all the fields in every node of the trie
• For a given prefix, traverse the trie till we reach the last character of the given prefix pre.
• Value of num field in the last node of string pre is the count of prefix in the given list of string.

Below is the implementation of the above approach:

## C++14

 `// C++ implementation of counting the``// number of words in a trie with a``// given prefix``#include "bits/stdc++.h"``using` `namespace` `std;` `// Trie Node``struct` `TrieNode {` `    ``// Using map to store the pointers``    ``// of children nodes for dynamic``    ``// implementation, for making the``    ``// program space efiicient``    ``map<``char``, TrieNode*> children;` `    ``// If isEndOfWord is true, then``    ``// node represents end of word``    ``bool` `isEndOfWord;` `    ``// num represents number of times``    ``// a character has appeared during``    ``// insertion of the words in the``    ``// trie``    ``map<``char``, ``int``> num;``};` `// Declare root node``struct` `TrieNode* root;` `// Function to create New Trie Node``struct` `TrieNode* getNewTrieNode()``{``    ``struct` `TrieNode* pNode = ``new` `TrieNode;``    ``pNode->isEndOfWord = ``false``;``    ``return` `pNode;``}` `// Function to insert a string in trie``void` `insertWord(string word)``{``    ``// To hold the value of root``    ``struct` `TrieNode* current = root;` `    ``// To hold letters of the word``    ``char` `s;` `    ``// Traverse through strings in list``    ``for` `(``int` `i = 0; i < word.length(); i++) {``        ``s = word[i];` `        ``// If s is not present in the``        ``// character field of current node``        ``if` `(current->children.find(s)``            ``== current->children.end()) {` `            ``// Get new node``            ``struct` `TrieNode* p = getNewTrieNode();` `            ``// Insert s in character``            ``// field of current node``            ``// with reference to node p``            ``(current->children)[s] = p;` `            ``// Insert s in num field``            ``// of current node with``            ``// value 1``            ``(current->num)[s] = 1;``        ``}``        ``else` `{` `            ``// Increment the count``            ``// corressponding to the``            ``// character s``            ``current->num[s] = (current->num)[s] + 1;``        ``}` `        ``// Go to next node``        ``current = (current->children)[s];``    ``}``    ``current->isEndOfWord = ``true``;``}` `// Function to count the number of``// words in trie with given prefix``int` `countWords(vector& words,``               ``string prefix)``{``    ``root = getNewTrieNode();` `    ``// Size of list of string``    ``int` `n = words.size();` `    ``// Construct trie containing``    ``// all the words``    ``for` `(``int` `i = 0; i < n; i++) {``        ``insertWord(words[i]);``    ``}` `    ``struct` `TrieNode* current = root;``    ``char` `s;` `    ``// Initialize the wordCount = 0``    ``int` `wordCount = 0;` `    ``for` `(``int` `i = 0; prefix[i]; i++) {``        ``s = prefix[i];` `        ``// If the complete prefix is``        ``// not present in the trie``        ``if` `(current->children.find(s)``            ``== current->children.end()) {` `            ``// Make wordCount 0 and``            ``// break out of loop``            ``wordCount = 0;``            ``break``;``        ``}` `        ``// Update the wordCount``        ``wordCount = (current->num)[s];` `        ``// Go to next node``        ``current = (current->children)[s];``    ``}` `    ``return` `wordCount;``}` `// Driver Code``int` `main()``{``    ``// input list of words``    ``vector words;``    ``words = { ``"apk"``, ``"app"``, ``"apple"``,``              ``"arp"``, ``"array"` `};` `    ``// Given prefix to find``    ``string prefix = ``"ap"``;` `    ``// Print the number of words with``    ``// given prefix``    ``cout << countWords(words, prefix);``    ``return` `0;``}`

## Python3

 `# Python3 implementation of counting the``# number of words in a trie with a``# given prefix`` ` `# Trie Node``class` `TrieNode:``    ` `    ``def` `__init__(``self``):``        ` `        ``# Using map to store the pointers``        ``# of children nodes for dynamic``        ``# implementation, for making the``        ``# program space efiicient``        ``self``.children ``=` `dict``()``    ` `        ``# If isEndOfWord is true, then``        ``# node represents end of word``        ``self``.isEndOfWord ``=` `False``    ` `        ``# num represents number of times``        ``# a character has appeared during``        ``# insertion of the words in the``        ``# trie``        ``self``.num ``=` `dict``()` `# Declare root node``root ``=` `None` `# Function to create New Trie Node``def` `getNewTrieNode():``    ` `    ``pNode ``=` `TrieNode()``    ``return` `pNode`` ` `# Function to insert a string in trie``def` `insertWord(word):``    ` `    ``global` `root``    ` `    ``# To hold the value of root``    ``current ``=` `root`` ` `    ``# To hold letters of the word``    ``s ``=` `''`` ` `    ``# Traverse through strings in list``    ``for` `i ``in` `range``(``len``(word)):``        ``s ``=` `word[i]`` ` `        ``# If s is not present in the``        ``# character field of current node``        ``if` `(s ``not` `in` `current.children):`` ` `            ``# Get new node``            ``p ``=` `getNewTrieNode()``            ` `            ``# Insert s in character``            ``# field of current node``            ``# with reference to node p``            ``current.children[s] ``=` `p`` ` `            ``# Insert s in num field``            ``# of current node with``            ``# value 1``            ``current.num[s] ``=` `1``        ``else``:`` ` `            ``# Increment the count``            ``# corressponding to the``            ``# character s``            ``current.num[s] ``=` `current.num[s] ``+` `1`` ` `        ``# Go to next node``        ``current ``=` `current.children[s]``    ` `    ``current.isEndOfWord ``=` `True` `# Function to count the number of``# words in trie with given prefix``def` `countWords(words, prefix):``    ` `    ``global` `root``    ``root ``=` `getNewTrieNode()`` ` `    ``# Size of list of string``    ``n ``=` `len``(words)`` ` `    ``# Construct trie containing``    ``# all the words``    ``for` `i ``in` `range``(n):``        ``insertWord(words[i])``        ` `    ``current ``=` `root``    ``s ``=` `''`` ` `    ``# Initialize the wordCount = 0``    ``wordCount ``=` `0``    ` `    ``for` `i ``in` `range``(``len``(prefix)):``        ``s ``=` `prefix[i]`` ` `        ``# If the complete prefix is``        ``# not present in the trie``        ``if` `(s ``not` `in` `current.children):`` ` `            ``# Make wordCount 0 and``            ``# break out of loop``            ``wordCount ``=` `0``            ``break``        ` `        ``# Update the wordCount``        ``wordCount ``=` `(current.num)[s]`` ` `        ``# Go to next node``        ``current ``=` `(current.children)[s]`` ` `    ``return` `wordCount` `# Driver Code``if` `__name__``=``=``'__main__'``:``    ` `    ``# input list of words``    ``words ``=` `[ ``"apk"``, ``"app"``, ``"apple"``,``              ``"arp"``, ``"array"` `]`` ` `    ``# Given prefix to find``    ``prefix ``=` `"ap"`` ` `    ``# Print the number of words with``    ``# given prefix``    ``print``(countWords(words, prefix))``    ` `# This code is contributed by rutvik_56`
Output:
`3`

Time Complexity: O(n*l) where n = number of words inserted in Trie and l = length of longest word inserted in Trie.

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