Longest Common Prefix using Sorting

Last Updated : 14 Nov, 2024

Given an array of strings arr[], the task is to return the longest common prefix among each and every strings present in the array. If there’s no prefix common in all the strings, return “”.

Examples:

Input: arr[] = [“geeksforgeeks”, “geeks”, “geek”, “geezer”]
Output: “gee”
Explanation: “gee” is the longest common prefix in all the given strings: “geeksforgeeks”, “geeks”, “geeks” and “geezer”.
Input: arr[] = [“apple”, “ape”, “april”]
Output : “ap”
Explanation: “ap” is the longest common prefix in all the given strings: “apple”, “ape” and “april”.
Input: arr[] = [“hello”, “world”]
Output: “”
Explanation: There’s no common prefix in the given strings.

Approach:

The idea is to sort the array of strings and find the common prefix of the first and last string of the sorted array. Sorting is used in this approach because it makes it easier to find the longest common prefix. When we sort the strings, the first and last strings in the sorted list will be the most different from each other in terms of their characters. So, the longest common prefix for all the strings must be a prefix of both the first and the last strings in the sorted list.

Illustration:

Given array of strings is [“geeksforgeeks”, “geeks”, “geek”, “geezer”].
After sorting it becomes [“geek” ,”geeks” ,”geeksforgeeks” ,”geezer”].
Now, to find the longest common prefix, we only need to compare the first and last strings (“geek” and “geezer“) because any common prefix between these two will also be a prefix for all the strings in between.
In this case, the common prefix between “geek” and “geezer” is “gee“, which is the longest common prefix for all the strings.

C++

// C++ program to find the longest common prefix
// using Sorting
#include <iostream>
#include <vector>
#include <algorithm>
using namespace std;

// Function to find the longest common prefix
string longestCommonPrefix(vector<string>& arr) {

    // Sort the vector of strings
    sort(arr.begin(), arr.end());

    // Compare the first and last strings
    // in the sorted list
    string first = arr.front();
    string last = arr.back();
    int minLength = min(first.size(), last.size());

    int i = 0;
  
    // Find the common prefix between the first
    // and last strings
    while (i < minLength && first[i] == last[i]) {
        i++;
    }

    // Return the common prefix
    return first.substr(0, i);
}

int main() {
    vector<string> arr = {"geeksforgeeks", "geeks",
                           "geek", "geezer"};
    cout << longestCommonPrefix(arr) << endl;

    return 0;
}

Java

// Java program to find the longest common prefix
// using Sorting
import java.util.Arrays;
class GfG {
  
    static String longestCommonPrefix(String[] arr){
      
        // Sort the array of strings
        Arrays.sort(arr);

        // Get the first and last strings after sorting
        String first = arr[0];
        String last = arr[arr.length - 1];
        int minLength = Math.min(first.length(), 
                                 	last.length());
        
        // Find the common prefix between the first 
      	// and last strings
      	int i = 0;
        while (i < minLength && 
               first.charAt(i) == last.charAt(i)) {
            i++;
        }

        // Return the common prefix
        return first.substring(0, i);
    }

    public static void main(String[] args){
        String[] arr = { "geeksforgeeks", "geeks", 
                        		"geek", "geezer" };
        System.out.println(longestCommonPrefix(arr));
    }
}

Python

# Python program to find the longest common prefix
# using Sorting

def longestCommonPrefix(arr):

    # Sort the list of strings
    arr.sort()

    # Get the first and last strings after sorting
    first = arr[0]
    last = arr[-1]
    minLength = min(len(first), len(last))

    i = 0
    # Find the common prefix between the first
    # and last strings
    while i < minLength and first[i] == last[i]:
        i += 1

    # Return the common prefix
    return first[:i]

if __name__ == "__main__":
    arr = ["geeksforgeeks", "geeks", "geek", "geezer"]
    print( longestCommonPrefix(arr))

// C# program to find the longest common prefix
// using Sorting
using System;

class GfG {
    static string LongestCommonPrefix(string[] arr){
     	
        // Sort the array of strings
        Array.Sort(arr);

        // Get the first and last strings after sorting
        string first = arr[0];
        string last = arr[arr.Length - 1];
        int minLength = Math.Min(first.Length, 
                                 		last.Length);

        int i = 0;
        // Find the common prefix between the first and 
      	// last strings
        while (i < minLength && first[i] == last[i]) {
            i++;
        }

        // Return the common prefix
        return first.Substring(0, i);
    }

    static void Main(){
        string[] arr = { "geeksforgeeks", "geeks", "geek",
                          "geezer" };
        Console.WriteLine(LongestCommonPrefix(arr));
    }
}

JavaScript

// JavaScript program to find the longest common prefix
// using Sorting

function longestCommonPrefix(arr){

    // Sort the array of strings
    arr.sort();

    // Get the first and last strings after sorting
    let first = arr[0];
    let last = arr[arr.length - 1];
    let minLength = Math.min(first.length, last.length);

    let i = 0;
    
    // Find the common prefix between the first and 
    // last strings
    while (i < minLength && first[i] === last[i]) {
        i++;
    }

    // Return the common prefix
    return first.substring(0, i);
}

// Driver Code
let arr = ["geeksforgeeks", "geeks", "geek", "geezer"];
console.log(longestCommonPrefix(arr) );

Output

gee

Time Complexity: O(n*m*log n), to sort the array, where n is the number of strings and m is the length of longest string.
Auxiliary Space: O(m) to store the strings first, last and result.

Other Approaches