Longest string in an array which matches with prefix of the given string
Last Updated :
11 Jul, 2025
Given an array of strings arr[] and Q queries where each query consists of a string str, the task is to find the longest string in the array that matches with prefix of the given string str i.e. the string must be prefix of str.
Examples:
Input: arr[] = {"GeeksForGeeks", "GeeksForGeeksd", "Arnab", "Art"}, q[] = {"GeeksForGeeks", "Ar", "Art"}
Output: GeeksForGeeks -1 Art
Input: arr[] = {"Geek", "Geek", "Geekss", "Geekk"}, q[] = {"Geek", "Geeks", "Geekk", "Gee"}
Output: Geek -1 Geekk -1
Naive approach: For every given string traverse through the array and check for the string which matches with prefix of the given string and store and print the longest string. Efficient Approach: This problem can be solved using a Trie. We will form an trie and insert the strings of the array in the trie and find the longest string that completely matches with the prefix of the given string. Below is the implementation of the above approach:
CPP
// C++ implementation of the approach
#include <bits/stdc++.h>
using namespace std;
const int ALPHABET_SIZE = 26;
// Trie node
struct TrieNode {
struct TrieNode* children[ALPHABET_SIZE];
// isEndOfWord is true if the node represents
// end of a word
bool isEndOfWord;
};
// Returns new trie node (initialized to NULLs)
struct TrieNode* getNode(void)
{
struct TrieNode* pNode = new TrieNode;
pNode->isEndOfWord = 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, string key)
{
struct TrieNode* pCrawl = root;
for (int i = 0; i < key.length(); i++) {
int index = key[i] - 'a';
if (!pCrawl->children[index])
pCrawl->children[index] = getNode();
if (i == key.length() - 1)
// Mark last node as leaf
pCrawl->children[index]->isEndOfWord = true;
pCrawl = pCrawl->children[index];
}
}
string getString(char x)
{
// string class has a constructor
// that allows us to specify size of
// string as first parameter and character
// to be filled in given size as second
// parameter.
string s(1, x);
return s;
}
// Function to return the
// longest required string
string search(struct TrieNode* root, string key)
{
struct TrieNode* pCrawl = root;
// Prefix of the string
string s = "";
for (int i = 0; i < key.length(); i++) {
int index = key[i] - 'a';
if (!pCrawl->children[index])
break;
s += getString((char)key[i]);
pCrawl = pCrawl->children[index];
}
if (pCrawl->isEndOfWord)
return s;
return "-1";
}
// Driver code
int main()
{
string arr[] = { "Geeks", "Geek",
"Geekss", "Geekks" };
int n = sizeof(arr) / sizeof(string);
// Construct trie
struct TrieNode* root = getNode();
for (int i = 0; i < n; i++)
insert(root, arr[i]);
string queries[] = { "Geek", "Geeks", "Geekk", "Gee" };
int q = sizeof(queries) / sizeof(string);
for (int i = 0; i < q; i++)
cout << search(root, queries[i]) << endl;
return 0;
}
import java.util.ArrayList;
// Trie node
class TrieNode {
TrieNode[] children = new TrieNode[26];
// isEndOfWord is true if the node represents
// end of a word
boolean isEndOfWord;
TrieNode() {
isEndOfWord = false;
for (int i = 0; i < 26; i++)
children[i] = null;
}
}
public class Trie {
TrieNode root;
Trie() {
root = new TrieNode();
}
// If not present, inserts key into trie
// If the key is prefix of trie node, just
// marks leaf node
void insert(String key) {
int n = key.length();
TrieNode pCrawl = root;
for (int i = 0; i < n; i++) {
int index = key.charAt(i) - 'a';
if (pCrawl.children[index] == null)
pCrawl.children[index] = new TrieNode();
pCrawl = pCrawl.children[index];
}
//Making True because we have reached to end
pCrawl.isEndOfWord = true;
}
// Function to return the
// longest required string
String search(String key) {
int n = key.length();
TrieNode pCrawl = root;
StringBuilder s = new StringBuilder();
for (int i = 0; i < n; i++) {
int index = key.charAt(i) - 'a';
if (pCrawl.children[index] == null)
break;
s.append(key.charAt(i));
pCrawl = pCrawl.children[index];
}
if (pCrawl != null && pCrawl.isEndOfWord)
return s.toString();
return "-1";
}
public static void main(String[] args) {
Trie trie = new Trie();
String[] arr = { "geeks", "geek", "geekss", "geekks" };
int n = arr.length;
// Inserting every string into trie
for (int i = 0; i < n; i++)
trie.insert(arr[i]);
String[] queries = { "geek", "geeks", "geekk", "gee" };
int q = queries.length;
for (int i = 0; i < q; i++)
System.out.println(trie.search(queries[i]));
}
}
# Python implementation of the approach
# Trie node
class TrieNode:
def __init__(self):
self.children = [None] * 26
# isEndOfWord is true if the node represents
# end of a word
self.is_end_of_word = False
# Returns new trie node (initialized to None)
def get_node():
node = TrieNode()
return node
# If not present, inserts key into trie
# If the key is prefix of trie node, just
# marks leaf node
def insert(root, key):
crawl = root
for i in range(len(key)):
index = ord(key[i]) - ord('a')
if not crawl.children[index]:
crawl.children[index] = get_node()
if i == len(key) - 1:
crawl.children[index].is_end_of_word = True
crawl = crawl.children[index]
# Function to return the
# longest required string
def search(root, key):
crawl = root
# Prefix of the string
s = ""
for i in range(len(key)):
index = ord(key[i]) - ord('a')
if not crawl.children[index]:
break
s += key[i]
crawl = crawl.children[index]
if crawl and crawl.is_end_of_word:
return s
return "-1"
# Driver code
def main():
arr = ["Geeks", "Geek", "Geekss", "Geekks"]
n = len(arr)
# Construct trie
root = get_node()
for i in range(n):
insert(root, arr[i])
queries = ["Geek", "Geeks", "Geekk", "Gee"]
q = len(queries)
for i in range(q):
print(search(root, queries[i]))
if __name__ == "__main__":
main()
# This code is contributed by Aman Kumar.
// C# code for the above approach
using System;
using System.Collections.Generic;
// Trie node
class TrieNode {
public TrieNode[] children = new TrieNode[26];
// isEndOfWord is true if the node represents
// end of a word
public bool isEndOfWord;
public TrieNode() {
isEndOfWord = false;
for (int i = 0; i < 26; i++)
children[i] = null;
}
}
public class Trie {
private TrieNode root;
public Trie() {
root = new TrieNode();
}
// If not present, inserts key into trie
// If the key is prefix of trie node, just
// marks leaf node
public void Insert(string key) {
int n = key.Length;
TrieNode pCrawl = root;
for (int i = 0; i < n; i++) {
int index = key[i] - 'a';
if (pCrawl.children[index] == null)
pCrawl.children[index] = new TrieNode();
pCrawl = pCrawl.children[index];
}
//Making True because we have reached to end
pCrawl.isEndOfWord = true;
}
// Function to return the
// longest required string
public string Search(string key) {
int n = key.Length;
TrieNode pCrawl = root;
System.Text.StringBuilder s = new System.Text.StringBuilder();
for (int i = 0; i < n; i++) {
int index = key[i] - 'a';
if (pCrawl.children[index] == null)
break;
s.Append(key[i]);
pCrawl = pCrawl.children[index];
}
if (pCrawl != null && pCrawl.isEndOfWord)
return s.ToString();
return "-1";
}
public static void Main(string[] args) {
Trie trie = new Trie();
string[] arr = { "geeks", "geek", "geekss", "geekks" };
int n = arr.Length;
// Inserting every string into trie
for (int i = 0; i < n; i++)
trie.Insert(arr[i]);
string[] queries = { "geek", "geeks", "geekk", "gee" };
int q = queries.Length;
for (int i = 0; i < q; i++)
Console.WriteLine(trie.Search(queries[i]));
}
}
// this code is contributed by bhardwajji
// Trie node
class TrieNode {
constructor() {
this.children = Array(26).fill(null);
// isEndOfWord is true if the node represents end of a word
this.is_end_of_word = false;
}
}
// Returns new trie node (initialized to null)
function get_node() {
const node = new TrieNode();
return node;
}
// If not present, inserts key into trie
// If the key is prefix of trie node, just marks leaf node
function insert(root, key) {
let crawl = root;
for (let i = 0; i < key.length; i++) {
const index = key[i].charCodeAt(0) - 'a'.charCodeAt(0);
if (!crawl.children[index]) {
crawl.children[index] = get_node();
}
if (i === key.length - 1) {
crawl.children[index].is_end_of_word = true;
}
crawl = crawl.children[index];
}
}
// Function to return the longest required string
function search(root, key) {
let crawl = root;
// Prefix of the string
let s = "";
for (let i = 0; i < key.length; i++) {
const index = key[i].charCodeAt(0) - 'a'.charCodeAt(0);
if (!crawl.children[index]) {
break;
}
s += key[i];
crawl = crawl.children[index];
}
if (crawl && crawl.is_end_of_word) {
return s;
}
return "-1";
}
// Driver code
function main() {
const arr = ["Geeks", "Geek", "Geekss", "Geekks"];
const n = arr.length;
// Construct trie
const root = get_node();
for (let i = 0; i < n; i++) {
insert(root, arr[i]);
}
const queries = ["Geek", "Geeks", "Geekk", "Gee"];
const q = queries.length;
for (let i = 0; i < q; i++) {
console.log(search(root, queries[i]));
}
}
main();
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