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# Count of strings whose prefix match with the given string to a given length k

• Difficulty Level : Medium
• Last Updated : 02 Mar, 2020

Given an array of strings arr[] and given some queries where each query consists of a string str and an integer k. The task is to find the count of strings in arr[] whose prefix of length k matches with the k length prefix of str.

Examples:

Input: arr[] = {“abba”, “abbb”, “abbc”, “abbd”, “abaa”, “abca”}, str = “abbg”, k = 3
Output: 4
“abba”, “abbb”, “abbc” and “abbd” are the matching strings.

Input: arr[] = {“geeks”, “geeksforgeeks”, “forgeeks”}, str = “geeks”, k = 2
Output: 2

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

Prerequisite: Trie | (Insert and Search)

Approach: We will form a trie and insert all the strings in the trie and we will create another variable (frequency) for each node which will store the frequency of prefix of the given strings. Now to get the count of strings whose prefix matches with the given string to a given length k we will have to traverse the trie to the length k from the root, the frequency of the Node will give the count of such strings.

Below is the implementation of the above approach:

## C++

 `// C++ implementation of the approach``#include ``using` `namespace` `std;`` ` `// Trie node (considering only lowercase alphabets)``struct` `Node {``    ``Node* arr;``    ``int` `freq;``};`` ` `// Function to insert a node in the trie``Node* insert(string s, Node* root)``{``    ``int` `in;``    ``Node* cur = root;``    ``for` `(``int` `i = 0; i < s.length(); i++) {``        ``in = s[i] - ``'a'``;`` ` `        ``// If there is no node created then create one``        ``if` `(cur->arr[in] == NULL)``            ``cur->arr[in] = ``new` `Node();`` ` `        ``// Increase the frequency of the node``        ``cur->arr[in]->freq++;`` ` `        ``// Move to the next node``        ``cur = cur->arr[in];``    ``}`` ` `    ``// Return the updated root``    ``return` `root;``}`` ` `// Function to return the count of strings``// whose prefix of length k matches with the``// k length prefix of the given string``int` `find(string s, ``int` `k, Node* root)``{``    ``int` `in, count = 0;``    ``Node* cur = root;`` ` `    ``// Traverse the string``    ``for` `(``int` `i = 0; i < s.length(); i++) {``        ``in = s[i] - ``'a'``;`` ` `        ``// If there is no node then return 0``        ``if` `(cur->arr[in] == NULL)``            ``return` `0;`` ` `        ``// Else traverse to the required node``        ``cur = cur->arr[in];`` ` `        ``count++;`` ` `        ``// Return the required count``        ``if` `(count == k)``            ``return` `cur->freq;``    ``}``    ``return` `0;``}`` ` `// Driver code``int` `main()``{``    ``string arr[] = { ``"abba"``, ``"abbb"``, ``"abbc"``, ``"abbd"``, ``"abaa"``, ``"abca"` `};``    ``int` `n = ``sizeof``(arr) / ``sizeof``(string);`` ` `    ``Node* root = ``new` `Node();`` ` `    ``// Insert the strings in the trie``    ``for` `(``int` `i = 0; i < n; i++)``        ``root = insert(arr[i], root);`` ` `    ``// Query 1``    ``cout << find(``"abbg"``, 3, root) << endl;`` ` `    ``// Query 2``    ``cout << find(``"abg"``, 2, root) << endl;`` ` `    ``// Query 3``    ``cout << find(``"xyz"``, 2, root) << endl;`` ` `    ``return` `0;``}`

## Java

 `// Java implementation of the approach``class` `GFG ``{`` ` `    ``// Trie node (considering only lowercase alphabets)``    ``static` `class` `Node ``    ``{``        ``Node[] arr = ``new` `Node[``26``];``        ``int` `freq;``    ``};`` ` `    ``// Function to insert a node in the trie``    ``static` `Node insert(String s, Node root)``    ``{``        ``int` `in;``        ``Node cur = root;``        ``for` `(``int` `i = ``0``; i < s.length(); i++) ``        ``{``            ``in = s.charAt(i) - ``'a'``;`` ` `            ``// If there is no node created then create one``            ``if` `(cur.arr[in] == ``null``)``                ``cur.arr[in] = ``new` `Node();`` ` `            ``// Increase the frequency of the node``            ``cur.arr[in].freq++;`` ` `            ``// Move to the next node``            ``cur = cur.arr[in];``        ``}`` ` `        ``// Return the updated root``        ``return` `root;``    ``}`` ` `    ``// Function to return the count of Strings``    ``// whose prefix of length k matches with the``    ``// k length prefix of the given String``    ``static` `int` `find(String s, ``int` `k, Node root) ``    ``{``        ``int` `in, count = ``0``;``        ``Node cur = root;`` ` `        ``// Traverse the String``        ``for` `(``int` `i = ``0``; i < s.length(); i++) ``        ``{``            ``in = s.charAt(i) - ``'a'``;`` ` `            ``// If there is no node then return 0``            ``if` `(cur.arr[in] == ``null``)``                ``return` `0``;`` ` `            ``// Else traverse to the required node``            ``cur = cur.arr[in];`` ` `            ``count++;`` ` `            ``// Return the required count``            ``if` `(count == k)``                ``return` `cur.freq;``        ``}``        ``return` `0``;``    ``}`` ` `    ``// Driver code``    ``public` `static` `void` `main(String[] args) ``    ``{``        ``String arr[] = { ``"abba"``, ``"abbb"``, ``"abbc"``, ``                        ``"abbd"``, ``"abaa"``, ``"abca"` `};``        ``int` `n = arr.length;`` ` `        ``Node root = ``new` `Node();`` ` `        ``// Insert the Strings in the trie``        ``for` `(``int` `i = ``0``; i < n; i++)``            ``root = insert(arr[i], root);`` ` `        ``// Query 1``        ``System.out.print(find(``"abbg"``, ``3``, root) + ``"\n"``);`` ` `        ``// Query 2``        ``System.out.print(find(``"abg"``, ``2``, root) + ``"\n"``);`` ` `        ``// Query 3``        ``System.out.print(find(``"xyz"``, ``2``, root) + ``"\n"``);`` ` `    ``}``}`` ` `// This code is contributed by PrinciRaj1992`

## Python3

 `# Python3 implementation of the approach `` ` `# Trie node (considering only lowercase alphabets) ``class` `Node :``    ``def` `__init__(``self``):``        ``self``.arr ``=` `[``None``]``*``26``        ``self``.freq ``=` `0``     ` `class` `Trie:``     ` `    ``# Trie data structure class ``    ``def` `__init__(``self``): ``        ``self``.root ``=` `self``.getNode() `` ` `    ``def` `getNode(``self``): ``     ` `        ``# Returns new trie node (initialized to NULLs) ``        ``return` `Node()`` ` `    ``# Function to insert a node in the trie ``    ``def` `insert(``self``, s): ``         ` `        ``_in ``=` `0``        ``cur ``=` `self``.root ``        ``for` `i ``in` `range``(``len``(s)): ``            ``_in ``=` `ord``(s[i]) ``-` `ord``(``'a'``) `` ` `            ``# If there is no node created then create one ``            ``if` `not` `cur.arr[_in]:``                ``cur.arr[_in] ``=` `self``.getNode() `` ` `            ``# Increase the frequency of the node ``            ``cur.arr[_in].freq ``+``=` `1`` ` `            ``# Move to the next node ``            ``cur ``=` `cur.arr[_in] `` ` `    ``# Function to return the count of strings ``    ``# whose prefix of length k matches with the ``    ``# k length prefix of the given string ``    ``def` `find(``self``, s, k): ``         ` `        ``_in ``=` `0``        ``count ``=` `0``        ``cur ``=` `self``.root `` ` `        ``# Traverse the string ``        ``for` `i ``in` `range``(``len``(s)): ``            ``_in ``=` `ord``(s[i]) ``-` `ord``(``'a'``) `` ` `            ``# If there is no node then return 0 ``            ``if` `cur.arr[_in] ``=``=` `None``: ``                ``return` `0`` ` `            ``# Else traverse to the required node ``            ``cur ``=` `cur.arr[_in] `` ` `            ``count ``+``=` `1`` ` `            ``# Return the required count ``            ``if` `count ``=``=` `k: ``                ``return` `cur.freq ``        ``return` `0`` ` `# Driver code ``def` `main(): ``     ` `    ``arr ``=` `[ ``"abba"``, ``"abbb"``, ``"abbc"``, ``"abbd"``, ``"abaa"``, ``"abca"` `] ``    ``n ``=` `len``(arr)`` ` `    ``root ``=` `Trie(); `` ` `    ``# Insert the strings in the trie ``    ``for` `i ``in` `range``(n): ``        ``root.insert(arr[i]) `` ` `    ``# Query 1 ``    ``print``(root.find(``"abbg"``, ``3``)) `` ` `    ``# Query 2 ``    ``print``(root.find(``"abg"``, ``2``)) `` ` `    ``# Query 3 ``    ``print``(root.find(``"xyz"``, ``2``))`` ` `if` `__name__ ``=``=` `'__main__'``: ``    ``main() ``     ` `# This code is contributed by divyamohan123 ``    `

## C#

 `// C# implementation of the approach``using` `System;`` ` `class` `GFG ``{`` ` `    ``// Trie node (considering only lowercase alphabets)``    ``public` `class` `Node ``    ``{``        ``public` `Node[] arr = ``new` `Node;``        ``public` `int` `freq;``    ``};`` ` `    ``// Function to insert a node in the trie``    ``static` `Node insert(String s, Node root)``    ``{``        ``int` `iN;``        ``Node cur = root;``        ``for` `(``int` `i = 0; i < s.Length; i++) ``        ``{``            ``iN = s[i] - ``'a'``;`` ` `            ``// If there is no node created then create one``            ``if` `(cur.arr[iN] == ``null``)``                ``cur.arr[iN] = ``new` `Node();`` ` `            ``// Increase the frequency of the node``            ``cur.arr[iN].freq++;`` ` `            ``// Move to the next node``            ``cur = cur.arr[iN];``        ``}`` ` `        ``// Return the updated root``        ``return` `root;``    ``}`` ` `    ``// Function to return the count of Strings``    ``// whose prefix of length k matches with the``    ``// k length prefix of the given String``    ``static` `int` `find(String s, ``int` `k, Node root) ``    ``{``        ``int` `iN, count = 0;``        ``Node cur = root;`` ` `        ``// Traverse the String``        ``for` `(``int` `i = 0; i < s.Length; i++) ``        ``{``            ``iN = s[i] - ``'a'``;`` ` `            ``// If there is no node then return 0``            ``if` `(cur.arr[iN] == ``null``)``                ``return` `0;`` ` `            ``// Else traverse to the required node``            ``cur = cur.arr[iN];`` ` `            ``count++;`` ` `            ``// Return the required count``            ``if` `(count == k)``                ``return` `cur.freq;``        ``}``        ``return` `0;``    ``}`` ` `    ``// Driver code``    ``public` `static` `void` `Main(String[] args) ``    ``{``        ``String []arr = { ``"abba"``, ``"abbb"``, ``"abbc"``, ``                        ``"abbd"``, ``"abaa"``, ``"abca"` `};``        ``int` `n = arr.Length;`` ` `        ``Node root = ``new` `Node();`` ` `        ``// Insert the Strings in the trie``        ``for` `(``int` `i = 0; i < n; i++)``            ``root = insert(arr[i], root);`` ` `        ``// Query 1``        ``Console.Write(find(``"abbg"``, 3, root) + ``"\n"``);`` ` `        ``// Query 2``        ``Console.Write(find(``"abg"``, 2, root) + ``"\n"``);`` ` `        ``// Query 3``        ``Console.Write(find(``"xyz"``, 2, root) + ``"\n"``);`` ` `    ``}``}`` ` `// This code is contributed by 29AjayKumar`
Output:
```4
6
0
```

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