Longest Common Prefix using Linked List

Given a set of strings, find the longest common prefix.

Examples:

Input  : {“geeksforgeeks”, “geeks”, “geek”, “geezer”}
Output : "gee"

Input  : {"apple", "ape", "april"}
Output : "ap"

Previous Approaches: Word by Word Matching, Character by Character Matching, Divide and Conquer, Binary Search, Using Trie Data Structure

Below is an algorithm to solve above problem using Linked List.

• Create a linked list using the characters of the first string as the data in the linked list.
• Then one by one using all the remaining strings, iterate over the linked list deleting all the nodes after the point where the string gets exhausted or linked list gets exhausted or the characters do not match.
• The remaining data in linked list is the required longest common prefix.

Below is the implementation in C++

// C++ Program to find the longest common prefix
// in an array of strings
#include <bits/stdc++.h>
using namespace std;

// Structure for a node in the linked list
struct Node {
    char data;
    Node* next;
};

// Function to print the data in the linked list
// Remaining nodes represent the longest common prefix
void printLongestCommonPrefix(Node* head)
{
    // If the linked list is empty there is
    // no common prefix
    if (head == NULL) {
        cout << "There is no common prefix\n";
        return;
    }

    // Printing the longest common prefix
    cout << "The longest common prefix is ";
    while (head != NULL) {
        cout << head->data;
        head = head->next;
    }
}

// Function that creates a linked list of characters
// for the first word in the array of strings
void Initialise(Node** head, string str)
{
    // We calculate the length of the string
    // as we will insert from the last to the
    // first character
    int l = str.length();

    // Inserting all the nodes with the characters
    // using insert at the beginning technique
    for (int i = l - 1; i >= 0; i--) {
        Node* temp = new Node;
        temp->data = str[i];
        temp->next = *head;
        *head = temp;
    }

    // Since we have passed the address of the
    // head node it is not required to return
    // anything
}

// Function to delete all the nodes
// from the unmatched node till the end of the
// linked list
void deleteNodes(Node* head)
{
    // temp is used to facilitate the deletion of nodes
    Node* current = head;
    Node* next;
    while (current != NULL) {
        next = current->next;
        free(current);
        current = next;
    }
}

// Function that compares the character of the string with
// the nodes of the linked list and deletes all nodes after
// the characters that do not match
void longestCommonPrefix(Node** head, string str)
{
    int i = 0;

    // Use the pointer to the previous node to
    // delete the link between the unmatched node
    // and its prev node
    Node* temp = *head;
    Node* prev = *head;
    while (temp != NULL) {

        // If the current string finishes or if the
        // the characters in the linked list do not match
        // with the character at the corresponding position
        // delete all the nodes after that.
        if (str[i] == '\0' || temp->data != str[i]) {

            // If the first node does not match then there
            // is no common prefix
            if (temp == *head) {
                free(temp);
                *head = NULL;
            }

            // Delete all the nodes starting from the
            // unmatched node
            else {
                prev->next = NULL;
                deleteNodes(temp);
            }
            break;
        }

        // If the character matches, move to next
        // node and store the address of the current
        // node in prev
        prev = temp;
        temp = temp->next;
        i++;
    }
}

int main()
{
    string arr[] = { "geeksforgeeks", "geeks", "geek", "geezer",
                     "geekathon" };
    int n = sizeof(arr) / sizeof(arr[0]);

    struct Node* head = NULL;

    // A linked list is created with all the characters
    // of the first string
    Initialise(&head, arr[0]);

    // Process all the remaining strings to find the
    // longest common prefix
    for (int i = 1; i < n; i++)
        longestCommonPrefix(&head, arr[i]);

    printLongestCommonPrefix(head);
}

Output:

The longest common prefix is gee