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Longest Common Prefix using Character by Character Matching

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

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

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

We have discussed word by word matching algorithm in previous post.
In this algorithm, instead of going through the strings one by one, we will go through the characters one by one. 
We consider our strings as – “geeksforgeeks”, “geeks”, “geek”, “geezer”.

longest_common_prefix2

 

longest_common_prefix3

longest_common_prefix4

 

longest_common_prefix5

Below is the implementation of this approach.

C++

//  A C++ Program to find the longest common prefix
#include<bits/stdc++.h>
using namespace std;
 
// A Function to find the string having the minimum
// length and returns that length
int findMinLength(string arr[], int n)
{
    int min = arr[0].length();
 
    for (int i=1; i<n; i++)
        if (arr[i].length() < min)
            min = arr[i].length();
 
    return(min);
}
 
// A Function that returns the longest common prefix
// from the array of strings
string commonPrefix(string arr[], int n)
{
    int minlen = findMinLength(arr, n);
 
    string result; // Our resultant string
    char current;  // The current character
 
    for (int i=0; i<minlen; i++)
    {
        // Current character (must be same
        // in all strings to be a part of
        // result)
        current = arr[0][i];
 
        for (int j=1 ; j<n; j++)
            if (arr[j][i] != current)
                return result;
 
        // Append to result
        result.push_back(current);
    }
 
    return (result);
}
 
// Driver program to test above function
int main()
{
    string arr[] = {"geeksforgeeks", "geeks",
                    "geek", "geezer"};
    int n = sizeof (arr) / sizeof (arr[0]);
 
    string ans = commonPrefix (arr, n);
 
    if (ans.length())
        cout << "The longest common prefix is "
             << ans;
    else
        cout << "There is no common prefix";
    return (0);
}

                    

Java

// A Java Program to find the longest common prefix
class GFG
{
 
    // A Function to find the string having the minimum
    // length and returns that length
    static int findMinLength(String arr[], int n)
    {
        int min = arr[0].length();
 
        for (int i = 1; i < n; i++)
        {
            if (arr[i].length() < min)
            {
                min = arr[i].length();
            }
        }
 
        return (min);
    }
 
    // A Function that returns the longest common prefix
    // from the array of strings
    static String commonPrefix(String arr[], int n)
    {
        int minlen = findMinLength(arr, n);
 
        String result = ""; // Our resultant string
        char current; // The current character
 
        for (int i = 0; i < minlen; i++)
        {
            // Current character (must be same
            // in all strings to be a part of
            // result)
            current = arr[0].charAt(i);
 
            for (int j = 1; j < n; j++)
            {
                if (arr[j].charAt(i) != current)
                {
                    return result;
                }
            }
 
            // Append to result
            result += (current);
        }
 
        return (result);
    }
 
    // Driver program to test above function
    public static void main(String[] args)
    {
        String arr[] = {"geeksforgeeks", "geeks",
            "geek", "geezer"};
        int n = arr.length;
 
        String ans = commonPrefix(arr, n);
 
        if (ans.length() > 0) {
            System.out.println("The longest common prefix is "
                    + ans);
        } else {
            System.out.println("There is no common prefix");
        }
    }
}
 
// This code contributed by Rajput-Ji

                    

Python 3

# Python 3 Program to find the longest common prefix
  
# A Function to find the string having the minimum
# length and returns that length
def findMinLength(arr, n):
 
    min = len(arr[0])
  
    for i in range(1,n):
        if (len(arr[i])< min):
            min = len(arr[i])
  
    return(min)
  
# A Function that returns the longest common prefix
# from the array of strings
def commonPrefix(arr, n):
 
    minlen = findMinLength(arr, n)
    result =""
    for i in range(minlen):
     
        # Current character (must be same
        # in all strings to be a part of
        # result)
        current = arr[0][i]
  
        for j in range(1,n):
            if (arr[j][i] != current):
                return result
  
        # Append to result
        result = result+current
  
    return (result)
  
# Driver program to test above function
if __name__ == "__main__":
     
    arr = ["geeksforgeeks", "geeks",
                    "geek", "geezer"]
    n = len(arr)
  
    ans = commonPrefix (arr, n)
  
    if (len(ans)):
        print("The longest common prefix is ",ans)
    else:
        print("There is no common prefix")

                    

C#

// A C# Program to find the longest common prefix
using System;
     
class GFG
{
 
    // A Function to find the string having the minimum
    // length and returns that length
    static int findMinLength(String []arr, int n)
    {
        int min = arr[0].Length;
 
        for (int i = 1; i < n; i++)
        {
            if (arr[i].Length < min)
            {
                min = arr[i].Length;
            }
        }
 
        return (min);
    }
 
    // A Function that returns the longest common prefix
    // from the array of strings
    static String commonPrefix(String []arr, int n)
    {
        int minlen = findMinLength(arr, n);
 
        String result = ""; // Our resultant string
        char current; // The current character
 
        for (int i = 0; i < minlen; i++)
        {
            // Current character (must be same
            // in all strings to be a part of
            // result)
            current = arr[0][i];
 
            for (int j = 1; j < n; j++)
            {
                if (arr[j][i] != current)
                {
                    return result;
                }
            }
 
            // Append to result
            result += (current);
        }
 
        return (result);
    }
 
    // Driver code
    public static void Main(String[] args)
    {
        String []arr = {"geeksforgeeks", "geeks",
            "geek", "geezer"};
        int n = arr.Length;
 
        String ans = commonPrefix(arr, n);
 
        if (ans.Length > 0)
        {
            Console.WriteLine("The longest common prefix is "
                    + ans);
        }
        else
        {
            Console.WriteLine("There is no common prefix");
        }
    }
}
 
/* This code contributed by PrinciRaj1992 */

                    

Javascript

<script>
// A Javascript Program to find the longest common prefix
 
    // A Function to find the string having the minimum
    // length and returns that length
    function findMinLength(arr,n)
    {
        let min = arr[0].length;
        for (let i = 1; i < n; i++)
        {
            if (arr[i].length < min)
            {
                min = arr[i].length;
            }
        }
   
        return (min);
    }
     
    // A Function that returns the longest common prefix
    // from the array of strings
    function commonPrefix(arr,n)
    {
        let minlen = findMinLength(arr, n);
        let result = ""; // Our resultant string
        let current; // The current character
        for (let i = 0; i < minlen; i++)
        {
            // Current character (must be same
            // in all strings to be a part of
            // result)
            current = arr[0][i];
   
            for (let j = 1; j < n; j++)
            {
                if (arr[j][i] != current)
                {
                    return result;
                }
            }
   
            // Append to result
            result += (current);
        }
   
        return (result);
    }
     
    // Driver program to test above function
    let arr=["geeksforgeeks", "geeks",
            "geek", "geezer"]
    let n = arr.length;
     
    let ans = commonPrefix(arr, n);
    if (ans.length > 0) {
        document.write("The longest common prefix is "
                + ans);
    } else {
        document.write("There is no common prefix");
    }
     
    //  This code is contributed by avanitrachhadiya2155
</script>

                    

Output
The longest common prefix is gee

How is this algorithm better than the “Word by Word Matching” algorithm ?-

In Set 1 we discussed about the “Word by Word Matching” Algorithm. 
Suppose you have the input strings as- “geeksforgeeks”, “geeks”, “geek”, “geezer”, “x”.
Now there is no common prefix string of the above strings. By the “Word by Word Matching” algorithm discussed in Set 1, we come to the conclusion that there is no common prefix string by traversing all the strings. But if we use this algorithm, then in the first iteration itself we will come to know that there is no common prefix string, as we don’t go further to look for the second character of each strings. 

This algorithm has a huge advantage when there are too many strings.

Time Complexity : Since we are iterating through all the characters of all the strings, so we can say that the time complexity is O(N M) where, 

N = Number of strings
M = Length of the Smallest string 

Auxiliary Space : To store the longest prefix string we are allocating space which is O(M).

Method 3(using some in-built C++ STL function):

 In this approach first of all we will find the string with smallest length. Then we will search in all other strings if this smallest string is present in them as a prefix or not. If in all strings this small string is present is present then we will print this string else we will keep reducing the length of the smallest string by one until its length becomes zero.

Below is the implementation of the above Approach:

Implementation:

C++

// C++ program to find the longest common
// prefix.
#include <bits/stdc++.h>
using namespace std;
// function to find the smallest string
// among all string
int shortest_string(string s[], int n)
{
    int minlength = INT_MAX, min_index;
    for (int i = 0; i < n; i++) {
        if (s[i].length() < minlength) {
            minlength = s[i].length();
            min_index = i;
        }
    }
    return min_index;
}
// function to find longest common
// prefix among all strings.
string findprefix(string s[], int n)
{
    // index of the smallest string
    int shortest_string_index = shortest_string(s, n);
 
    while (s[shortest_string_index].length() > 0) {
 
        int count = 0;
        for (int i = 0; i < n; i++) {
            // checking whether all strings have prefix
            // which is equal to smallest string
            if (s[i].find(s[shortest_string_index]) == 0) {
                count++;
            }
        }
        // checking that all the string's
        // prefix is equal to smallest string
        // or not.
        if (count == n) {
            cout << "longest common prefix is: " << endl;
            return s[shortest_string_index];
            break;
        }
        // deleting the last character
        // of the smallest string.
        s[shortest_string_index].pop_back();
    }
 
    return "no common prefix among all strings";
}
// driver code
int main()
{
    string s[]
        = { "geeksforgeeks", "geeks", "geek", "geezer" };
    int n = sizeof(s) / sizeof(s[0]);
    // function call
    cout << findprefix(s, n);
    return 0;
}
 
// this code is contributed by Machhaliya Mohammad.

                    

Java

/*package whatever //do not write package name here */
import java.io.*;
 
class GFG {
 
  // function to find the smallest string
  // among all string
  static int shortest_string(String s[], int n)
  {
    int minlength = Integer.MAX_VALUE, min_index = -1;
    for (int i = 0; i < n; i++) {
      if (s[i].length() < minlength) {
        minlength = s[i].length();
        min_index = i;
      }
    }
    return min_index;
  }
  // function to find longest common
  // prefix among all strings.
  static String findprefix(String s[], int n)
  {
    // index of the smallest string
    int shortest_string_index = shortest_string(s, n);
 
    while (s[shortest_string_index].length() > 0) {
 
      int count = 0;
      for (int i = 0; i < n; i++) {
        // checking whether all strings have prefix
        // which is equal to smallest string
        if (s[i].indexOf(s[shortest_string_index]) == 0) {
          count++;
        }
      }
      // checking that all the string's
      // prefix is equal to smallest string
      // or not.
      if (count == n) {
        System.out.println("longest common prefix is:");
        return s[shortest_string_index];
      }
      // deleting the last character
      // of the smallest string.
      s[shortest_string_index] = s[shortest_string_index].substring(0,s[shortest_string_index].length()-1);
    }
 
    return "no common prefix among all strings";
  }
 
  public static void main (String[] args) {
    String s[] = { "geeksforgeeks", "geeks", "geek", "geezer" };
    int n = s.length;
 
    // function call
    System.out.println(findprefix(s, n));
 
  }
}
 
// This code is contributed by aadityaburujwale.

                    

Python3

# Python program to find the longest common
# prefix.
import sys
 
# function to find the smallest string
# among all string
def shortest_string(s, n):
    minlength = sys.maxsize
    min_index = 0
    for i in range(n):
        if len(s[i]) < minlength:
            minlength = len(s[i])
            min_index = i
    return min_index
   
# function to find longest common
# prefix among all strings.
def findprefix(s, n):
   
    # index of the smallest string
    shortest_string_index = shortest_string(s, n)
    while len(s[shortest_string_index]) > 0:
        count = 0
        for i in range(n):
           
            # checking whether all strings have prefix
            # which is equal to smallest string
            if s[i].find(s[shortest_string_index]) == 0:
                count += 1
                 
        # checking that all the string's
        # prefix is equal to smallest string
        # or not.
        if count == n:
            print("longest common prefix is: ")
            return s[shortest_string_index]
            break
             
        # deleting the last character
        # of the smallest string.
        s[shortest_string_index] = s[shortest_string_index][:-1]
   
    return "no common prefix among all strings"
   
# driver code
s = ["geeksforgeeks", "geeks",
"geek", "geezer"]
n = len(s)
 
# function call
print(findprefix(s, n))
 
# This code is contributed by akashish__

                    

C#

// Include namespace system
using System;
 
public class GFG
{
   
  // function to find the smallest string
  // among all string
  public static int shortest_string(String[] s, int n)
  {
    var minlength = int.MaxValue;
    var min_index = -1;
    for (int i = 0; i < n; i++)
    {
      if (s[i].Length < minlength)
      {
        minlength = s[i].Length;
        min_index = i;
      }
    }
    return min_index;
  }
 
  // function to find longest common
  // prefix among all strings.
  public static String findprefix(String[] s, int n)
  {
 
    // index of the smallest string
    var shortest_string_index = GFG.shortest_string(s, n);
    while (s[shortest_string_index].Length > 0)
    {
      var count = 0;
      for (int i = 0; i < n; i++)
      {
 
        // checking whether all strings have prefix
        // which is equal to smallest string
        if (s[i].IndexOf(s[shortest_string_index]) == 0)
        {
          count++;
        }
      }
 
      // checking that all the string's
      // prefix is equal to smallest string
      // or not.
      if (count == n)
      {
        Console.WriteLine("longest common prefix is:");
        return s[shortest_string_index];
      }
 
      // deleting the last character
      // of the smallest string.
      s[shortest_string_index] = s[shortest_string_index].Substring(0,s[shortest_string_index].Length - 1-0);
    }
    return "no common prefix among all strings";
  }
  public static void Main(String[] args)
  {
    String[] s = {"geeksforgeeks", "geeks", "geek", "geezer"};
    var n = s.Length;
 
    // function call
    Console.WriteLine(GFG.findprefix(s, n));
  }
}
 
// This code is contributed by sourabhdalal0001.

                    

Javascript

// JavaScript program to find the longest common
// prefix.
function shortest_string(s, n) {
    let minlength = Number.MAX_SAFE_INTEGER;
    let min_index;
 
    for (let i = 0; i < n; i++) {
        if (s[i].length < minlength) {
            minlength = s[i].length;
            min_index = i;
        }
    }
 
    return min_index;
}
 
function findprefix(s, n) {
    // index of the smallest string
    let shortest_string_index = shortest_string(s, n);
 
    while (s[shortest_string_index].length > 0) {
        let count = 0;
        for (let i = 0; i < n; i++) {
            // checking whether all strings have prefix
            // which is equal to smallest string
            if (s[i].indexOf(s[shortest_string_index]) === 0) {
                count++;
            }
        }
        // checking that all the string's
        // prefix is equal to smallest string
        // or not.
        if (count === n) {
            console.log("longest common prefix is: ");
            return s[shortest_string_index];
            break;
        }
        // deleting the last character
        // of the smallest string.
        s[shortest_string_index] = s[shortest_string_index].slice(0, -1);
    }
 
    return "no common prefix among all strings";
}
 
// driver code
let s = ["geeksforgeeks", "geeks", "geek", "geezer"];
let n = s.length;
 
// function call
console.log(findprefix(s, n));
 
// This code is contributed by akashish__

                    

Output
longest common prefix is: 
gee

Time complexity: O(n*m) where n is total number of strings and m is length of the smallest string among all strings.
Auxiliary space: O(1)

 



Last Updated : 10 Feb, 2023
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