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Longest Common Prefix using Sorting

  • Difficulty Level : Medium
  • Last Updated : 27 Apr, 2021

Problem Statement: Given a set of strings, find the longest common prefix.
Examples: 
 

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

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

 

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The longest common prefix for an array of strings is the common prefix between 2 most dissimilar strings. For example, in the given array {“apple”, “ape”, “zebra”}, there is no common prefix because the 2 most dissimilar strings of the array “ape” and “zebra” do not share any starting characters. 
We have discussed five different approaches in below posts. 
 



  1. Word by Word Matching
  2. Character by Character Matching
  3. Divide and Conquer
  4. Binary Search.
  5. Using Trie)

 

In this post a new method based on sorting is discussed. The idea is to sort the array of strings and find the common prefix of the first and last string of the sorted array.
 

C++




// C++ program to find longest common prefix
// of given array of words.
#include<iostream>
#include<algorithm>
 
using namespace std;
 
// Function to find the longest common prefix
string longestCommonPrefix(string ar[], int n)
{
 
    // If size is 0, return empty string
    if (n == 0)
        return "";
 
    if (n == 1)
        return ar[0];
 
    // Sort the given array
    sort(ar, ar + n);
 
    // Find the minimum length from
    // first and last string
    int en = min(ar[0].size(),
                 ar[n - 1].size());
 
    // Now the common prefix in first and
    // last string is the longest common prefix
    string first = ar[0], last = ar[n - 1];
    int i = 0;
    while (i < en && first[i] == last[i])
        i++;
 
    string pre = first.substr(0, i);
    return pre;
}
 
// Driver Code
int main()
{
    string ar[] = {"geeksforgeeks", "geeks",
                           "geek", "geezer"};
    int n = sizeof(ar) / sizeof(ar[0]);
    cout << "The longest common prefix is: "
         << longestCommonPrefix(ar, n);
    return 0;
}
 
// This code is contributed by jrolofmeister

Java




// Java program to find longest common prefix of
// given array of words.
import java.util.*;
 
public class GFG
{
    public String longestCommonPrefix(String[] a)
    {
        int size = a.length;
 
        /* if size is 0, return empty string */
        if (size == 0)
            return "";
 
        if (size == 1)
            return a[0];
 
        /* sort the array of strings */
        Arrays.sort(a);
 
        /* find the minimum length from first and last string */
        int end = Math.min(a[0].length(), a[size-1].length());
 
        /* find the common prefix between the first and
           last string */
        int i = 0;
        while (i < end && a[0].charAt(i) == a[size-1].charAt(i) )
            i++;
 
        String pre = a[0].substring(0, i);
        return pre;
    }
 
    /* Driver Function to test other function */
    public static void main(String[] args)
    {
        GFG gfg = new GFG();
        String[] input = {"geeksforgeeks", "geeks", "geek", "geezer"};
        System.out.println( "The longest Common Prefix is : " +
                                   gfg.longestCommonPrefix(input));
    }
}

Python 3




# Python 3 program to find longest
# common prefix of given array of words.
def longestCommonPrefix( a):
     
    size = len(a)
 
    # if size is 0, return empty string
    if (size == 0):
        return ""
 
    if (size == 1):
        return a[0]
 
    # sort the array of strings
    a.sort()
     
    # find the minimum length from
    # first and last string
    end = min(len(a[0]), len(a[size - 1]))
 
    # find the common prefix between
    # the first and last string
    i = 0
    while (i < end and
           a[0][i] == a[size - 1][i]):
        i += 1
 
    pre = a[0][0: i]
    return pre
 
# Driver Code
if __name__ == "__main__":
 
    input = ["geeksforgeeks", "geeks",
                     "geek", "geezer"]
    print("The longest Common Prefix is :" ,
                 longestCommonPrefix(input))
 
# This code is contributed by ita_c

C#




// C# program to find longest common prefix of
// given array of words.
using System;
         
public class GFG {
     
    static string longestCommonPrefix(String[] a)
    {
        int size = a.Length;
 
        /* if size is 0, return empty string */
        if (size == 0)
            return "";
 
        if (size == 1)
            return a[0];
 
        /* sort the array of strings */
        Array.Sort(a);
 
        /* find the minimum length from first
        and last string */
        int end = Math.Min(a[0].Length,
                            a[size-1].Length);
 
        /* find the common prefix between the
        first and last string */
        int i = 0;
        while (i < end && a[0][i] == a[size-1][i] )
            i++;
 
        string pre = a[0].Substring(0, i);
        return pre;
    }
 
    /* Driver Function to test other function */
    public static void Main()
    {
         
        string[] input = {"geeksforgeeks", "geeks",
                                 "geek", "geezer"};
                                  
        Console.WriteLine( "The longest Common"
                              + " Prefix is : "
                  + longestCommonPrefix(input));
    }
}
 
// This code is contributed by Sam007.

Javascript




<script>
// Javascript program to find longest common prefix of
// given array of words.
     
    function longestCommonPrefix(a)
    {
        let size = a.length;
   
        /* if size is 0, return empty string */
        if (size == 0)
            return "";
   
        if (size == 1)
            return a[0];
   
        /* sort the array of strings */
        a.sort();
   
        /* find the minimum length from first and last string */
        let end = Math.min(a[0].length, a[size-1].length);
   
        /* find the common prefix between the first and
           last string */
        let i = 0;
        while (i < end && a[0][i] == a[size-1][i] )
            i++;
   
        let pre = a[0].substring(0, i);
        return pre;
    }
     
    /* Driver Function to test other function */
    let input=["geeksforgeeks", "geeks", "geek", "geezer"];
    document.write( "The longest Common Prefix is : " +
                                   longestCommonPrefix(input));
       
     
    // This code is contributed by rag2127
</script>

Output: 
 

The longest common prefix is : gee

Time Complexity: O(MAX * n * log n ) where n is the number of strings in the array and MAX is maximum number of characters in any string. Please note that comparison of two strings would take at most O(MAX) time and for sorting n strings, we would need O(MAX * n * log n ) time.
This article is contributed by Saloni Baweja. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
Please write comments if you find anything incorrect, or you want to share more information about the topic discussed above.
 




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