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Longest Common Prefix using Word by Word Matching

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

Examples: 

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

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

We start with an example. Suppose there are two strings- “geeksforgeeks” and “geeks”. What is the longest common prefix in both of them? It is “geeks”.
Now let us introduce another word “geek”. So now what is the longest common prefix in these three words ? It is “geek”
We can see that the longest common prefix holds the associative property, i.e- 

LCP(string1, string2, string3) 
         = LCP (LCP (string1, string2), string3)

Like here

LCP (“geeksforgeeks”, “geeks”, “geek”)
     =  LCP (LCP (“geeksforgeeks”, “geeks”), “geek”)
     =  LCP (“geeks”, “geek”) = “geek”

So we can make use of the above associative property to find the LCP of the given strings. We one by one calculate the LCP of each of the given string with the LCP so far. The final result will be our longest common prefix of all the strings.
Note that it is possible that the given strings have no common prefix. This happens when the first character of all the strings are not same.
We show the algorithm with the input strings- “geeksforgeeks”, “geeks”, “geek”, “geezer” by the below figure.

longest_common_prefix

Below is the implementation of above approach: 

C

#include <stdio.h>
#include <string.h>
 
char* commonPrefixUtil(char* str1, char* str2)
{
    char* result = (char*)malloc(100 * sizeof(char));
    int len = strlen(str1) < strlen(str2) ? strlen(str1)
                                          : strlen(str2);
 
    for (int i = 0; i < len; i++) {
        if (str1[i] != str2[i])
            break;
        result[i] = str1[i];
    }
 
    result[len] = '\0';
    return result;
}
 
char* commonPrefix(char* arr[], int n)
{
    char* prefix = arr[0];
 
    for (int i = 1; i < n; i++) {
        prefix = commonPrefixUtil(prefix, arr[i]);
    }
 
    return prefix;
}
 
int main()
{
    char* arr[]
        = { "geeksforgeeks", "geeks", "geek", "geezer" };
    int n = sizeof(arr) / sizeof(arr[0]);
 
    char* ans = commonPrefix(arr, n);
 
    if (strlen(ans))
        printf("The longest common prefix is - %s", ans);
    else
        printf("There is no common prefix");
 
    free(ans);
 
    return 0;
}

                    

C++

//  A C++ Program to find the longest common prefix
#include<bits/stdc++.h>
using namespace std;
 
// A Utility Function to find the common prefix between
// strings- str1 and str2
string commonPrefixUtil(string& str1, string& str2)
{
    string result = "";
    int len = min(str1.length(), str2.length());
 
    // Compare str1 and str2
    for (int i = 0; i < len; i++)
    {
        if (str1[i] != str2[i])
            break;
        result += str1[i];
    }
 
    return (result);
}
 
// A Function that returns the longest common prefix
// from the array of strings
string commonPrefix (string arr[], int n)
{
    string prefix =  arr[0];
 
    for (int i=1; i < n; i++)
        prefix = commonPrefixUtil(prefix, arr[i]);
 
    return (prefix);
}
 
// 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())
        printf ("The longest common prefix is - %s",
                 ans.c_str());
    else
        printf("There is no common prefix");
 
    return (0);
}

                    

Java

// Java Program to find the longest common prefix
 
class GFG {
 
// A Utility Function to find the common prefix between
// strings- str1 and str2
    static String commonPrefixUtil(String str1, String str2) {
        String result = "";
        int n1 = str1.length(), n2 = str2.length();
 
        // Compare str1 and str2
        for (int i = 0, j = 0; i <= n1 - 1 && j <= n2 - 1; i++, j++) {
            if (str1.charAt(i) != str2.charAt(j)) {
                break;
            }
            result += str1.charAt(i);
        }
 
        return (result);
    }
 
// A Function that returns the longest common prefix
// from the array of strings
    static String commonPrefix(String arr[], int n) {
        String prefix = arr[0];
 
        for (int i = 1; i <= n - 1; i++) {
            prefix = commonPrefixUtil(prefix, arr[i]);
        }
 
        return (prefix);
    }
 
// 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.printf("The longest common prefix is - %s",
                    ans);
        } else {
            System.out.printf("There is no common prefix");
        }
    }
}
// This code is contributed by 29AjayKumar

                    

Python3

# A python3 Program to find the longest
# common prefix
 
# A Utility Function to find the common
# prefix between strings- str1 and str2
def commonPrefixUtil(str1, str2):
 
    result = "";
    n1 = len(str1)
    n2 = len(str2)
 
    # Compare str1 and str2
    i = 0
    j = 0
    while i <= n1 - 1 and j <= n2 - 1:
     
        if (str1[i] != str2[j]):
            break
             
        result += str1[i]
        i += 1
        j += 1
 
    return (result)
 
# A Function that returns the longest
# common prefix from the array of strings
def commonPrefix (arr, n):
 
    prefix = arr[0]
 
    for i in range (1, n):
        prefix = commonPrefixUtil(prefix, arr[i])
 
    return (prefix)
 
# Driver Code
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")
 
# This code is contributed by ita_c

                    

C#

// C# Program to find the longest
// common prefix
using System;
 
class GFG
{
 
// A Utility Function to find
// the common prefix between
// strings- str1 and str2
static String commonPrefixUtil(String str1,
                               String str2)
{
    String result = "";
    int n1 = str1.Length,
        n2 = str2.Length;
 
    // Compare str1 and str2
    for (int i = 0, j = 0;
             i <= n1 - 1 && j <= n2 - 1;
             i++, j++)
    {
        if (str1[i] != str2[j])
        {
            break;
        }
        result += str1[i];
    }
 
    return (result);
}
 
// A Function that returns the longest
// common prefix from the array of strings
static String commonPrefix(String []arr, int n)
{
    String prefix = arr[0];
 
    for (int i = 1; i <= n - 1; i++)
    {
        prefix = commonPrefixUtil(prefix,
                     arr.GetValue(i).ToString());
    }
 
    return (prefix);
}
 
// Driver Code
public static void Main()
{
    String []arr = {"geeksforgeeks", "geeks",
                    "geek", "geezer"};
    int n = arr.Length;
 
    String ans = commonPrefix(arr, n);
 
    if (ans.Length > 0)
    {
        Console.Write("The longest common " +
                       "prefix is - " + ans);
    }
    else
    {
        Console.Write("There is no common prefix");
    }
}
}
 
// This code is contributed
// by 29AjayKumar

                    

Javascript

<script>
// Javascript Program to find the longest common prefix
     
    // A Utility Function to find the common prefix between
    // strings- str1 and str2
    function commonPrefixUtil(str1,str2)
    {
        let  result = "";
        let n1 = str1.length, n2 = str2.length;
        // Compare str1 and str2
        for (let i = 0, j = 0; i <= n1 - 1 && j <= n2 - 1; i++, j++) {
            if (str1[i] != str2[j]) {
                break;
            }
            result += str1[i];
        }
   
        return (result);
    }
     
    // A Function that returns the longest common prefix
    // from the array of strings
    function commonPrefix(arr,n)
    {
        let prefix = arr[0];
         
        for (let i = 1; i <= n - 1; i++) {
            prefix = commonPrefixUtil(prefix, arr[i]);
        }
   
        return (prefix);
    }
     
    // 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 rag2127
 
     
</script>

                    

Output
The longest common prefix is - gee

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

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

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

How to improve this ? 
Please see Longest Common Prefix | Set 2 (Character by Character Matching)

The approach is to sort the array of strings and compare the first and last elements of the sorted array. The common prefix between these two elements will be the longest common prefix for the entire array

Step-by-step algorithm for implementing the approach 

  1. Define the function longest_common_prefix(arr) that takes a list of strings as input.
  2. Check if the input list is empty. If it is, return an empty string.
  3. Sort the input list of strings using the sort() method.
  4. Initialize the prefix to an empty string.
  5. Loop through the characters of the first string in the sorted list using the range() function and the len() function to get the length of the string.
  6. Check if the current character is the same in all strings in the input list using a generator expression and the all() function.
  7. If the current character is the same in all strings, add it to the prefix using the += operator.
  8. If the current character is not the same in all strings, break the loop using the break statement.
  9. Return the prefix.
  10. Define a sample list of strings, and call the longest_common_prefix() function with the sample list as input.
  11. Print the result.

C

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
 
#define MAX_STR_LEN 100
 
char* longestCommonPrefix(char** arr, int n)
{
    // Check if the input list is empty
    if (n == 0) {
        // If the list is empty, return an empty string
        return "";
    }
 
    // Sort the input list of strings
    for (int i = 0; i < n; i++) {
        for (int j = i + 1; j < n; j++) {
            if (strcmp(arr[i], arr[j]) > 0) {
                char* temp = arr[i];
                arr[i] = arr[j];
                arr[j] = temp;
            }
        }
    }
 
    // Initialize the prefix to an empty string
    char* prefix
        = (char*)malloc(sizeof(char) * MAX_STR_LEN);
    prefix[0] = '\0';
 
    // Loop through the first string of the sorted list
    for (int i = 0; i < strlen(arr[0]); i++) {
        // Check if the current character is the same in all
        // strings in the list
        int match = 1;
        for (int j = 1; j < n; j++) {
            if (strlen(arr[j]) <= i
                || arr[j][i] != arr[0][i]) {
                match = 0;
                break;
            }
        }
 
        // If the current character is the same in all
        // strings, add it to the prefix
        if (match) {
            strncat(prefix, &arr[0][i], 1);
        }
        else {
            // If the current character is not the same in
            // all strings, break the loop
            break;
        }
    }
    // Return the prefix
    return prefix;
}
 
int main()
{
 
    // A sample list of strings
    char* arr[]
        = { "geeksforgeeks", "geeks", "geek", "geezer" };
    int n = sizeof(arr) / sizeof(arr[0]);
 
    // Print the longest common prefix
    printf("The longest common prefix is: %s\n",
           longestCommonPrefix(arr, n));
 
    return 0;
}

                    

C++

#include<bits/stdc++.h>
using namespace std;
 
//function to find longest common prefix
string longestCommonPrefix(vector<string> &arr){
   
  // Check if the input list is empty
  if (arr.size() == 0) {
    // If the list is empty, return an empty string
    return "";
  }
   
  // Sort the input list of strings
  sort(arr.begin(),arr.end());
 
  // Initialize the prefix to an empty string
  string prefix;
 
  // Loop through the first string of the sorted list
  for (int i = 0; i < arr[0].length(); i++) {
    // Check if the current character is the same in all strings in the list
    bool match = true;
    for (int j = 1; j < arr.size(); j++) {
      if (arr[j].length() <= i || arr[j][i] != arr[0][i]) {
        match = false;
        break;
      }
    }
 
    // If the current character is the same in all strings, add it to the prefix
    if (match) {
      prefix+=arr[0][i];
    }
    else{
      // If the current character is not the same in all strings, break the loop
      break;
    }
  }
  // Return the prefix
  return prefix;
}
int main() {
   
  // A sample list of strings
  vector<string> arr= { "geeksforgeeks", "geeks", "geek", "geezer" };
   
  // Print the longest common prefix
  cout<<"The longest common prefix is: "<<longestCommonPrefix(arr);
   
  return 0;
}
//This code is contributed by shubhamrajput6156

                    

Java

import java.util.Arrays;
 
public class Main {
    public static String longestCommonPrefix(String[] arr) {
        // Check if the input list is empty
        if (arr == null || arr.length == 0) {
            // If the list is empty, return an empty string
            return "";
        }
 
        // Sort the input list of strings
        Arrays.sort(arr);
 
        // Initialize the prefix to an empty string
        StringBuilder prefix = new StringBuilder();
 
        // Loop through the first string of the sorted list
        for (int i = 0; i < arr[0].length(); i++) {
            // Check if the current character is the same in all strings in the list
            boolean match = true;
            for (int j = 1; j < arr.length; j++) {
                if (arr[j].length() <= i || arr[j].charAt(i) != arr[0].charAt(i)) {
                    match = false;
                    break;
                }
            }
 
            // If the current character is the same in all strings, add it to the prefix
            if (match) {
                prefix.append(arr[0].charAt(i));
            } else {
                // If the current character is not the same in all strings, break the loop
                break;
            }
        }
 
        // Return the prefix
        return prefix.toString();
    }
 
    public static void main(String[] args) {
        // A sample list of strings
        String[] arr = { "geeksforgeeks", "geeks", "geek", "geezer" };
        // Print the longest common prefix
        System.out.println("The longest common prefix is: " + longestCommonPrefix(arr));
    }
}

                    

Python3

# A function to find the longest common prefix in a list of strings
def longest_common_prefix(arr):
    # Check if the input list is empty
    if not arr:
        # If the list is empty, return an empty string
        return ""
    # Sort the input list of strings
    arr.sort()
    # Initialize the prefix to an empty string
    prefix = ""
    # Loop through the first string of the sorted list
    for i in range(len(arr[0])):
        # Check if the current character is the same in all strings in the list
        if all(x[i] == arr[0][i] for x in arr):
            # If the current character is the same in all strings, add it to the prefix
            prefix += arr[0][i]
        else:
            # If the current character is not the same in all strings, break the loop
            break
    # Return the prefix
    return prefix
 
# A sample list of strings
arr = ["geeksforgeeks", "geeks", "geek", "geezer"]
# Print the longest common prefix
print("The longest common prefix is: ", longest_common_prefix(arr))

                    

C#

// C# Program to find the longest
// common prefix
using System;
using System.Linq;
 
class GFG {
    static String longestCommonPrefix(String[] arr)
    {
        // Check if the input list is empty
        if (arr == null || arr.Length == 0) {
            // If the list is empty, return an empty string
            return "";
        }
 
        // Sort the input list of strings
        Array.Sort(arr);
 
        // Initialize the prefix to an empty string
        string prefix = "";
        // Loop through the first string of the sorted list
        for (int i = 0; i < arr[0].Length; i++) {
            // Check if the current character is the same in
            // all strings in the list
            bool match = true;
            for (int j = 1; j < arr.Length; j++) {
                if (arr[j].Length <= i
                    || arr[j][i] != arr[0][i]) {
                    match = false;
                    break;
                }
            }
 
            // If the current character is the same in all
            // strings, add it to the prefix
            if (match) {
                prefix += arr[0][i];
            }
            else {
                // If the current character is not the same
                // in all strings, break the loop
                break;
            }
        }
 
        // Return the prefix
        return (prefix);
    }
 
    public static void Main(string[] args)
    {
        String[] arr = { "geeksforgeeks", "geeks", "geek",
                         "geezer" };
        // Print the longest common prefix
        Console.WriteLine("The longest common prefix is: "
                          + longestCommonPrefix(arr));
    }
}
 
//This Code is contributed by Akshay Tripathi(akshaytripathi19410)

                    

Javascript

function longestCommonPrefix(arr)
{
 
  // Check if the input list is empty
  if (!arr.length)
  {
   
    // If the list is empty, return an empty string
    return "";
  }
   
  // Sort the input list of strings
  arr.sort();
   
  // Initialize the prefix to an empty string
  let prefix = "";
   
  // Loop through the first string of the sorted list
  for (let i = 0; i < arr[0].length; i++)
  {
   
    // Check if the current character is the same in all strings in the list
    if (arr.every((x) => x[i] === arr[0][i]))
    {
     
      // If the current character is the same in all strings, add it to the prefix
      prefix += arr[0][i];
    }
    else {
     
      // If the current character is not the same in all strings, break the loop
      break;
    }
  }
   
  // Return the prefix
  return prefix;
}
 
// A sample list of strings
const arr = ["geeksforgeeks", "geeks", "geek", "geezer"];
 
// Print the longest common prefix
console.log("The longest common prefix is: ", longestCommonPrefix(arr));

                    

Output
The longest common prefix is:  gee


Time complexity: O(NMlogM), where N is the number of strings in the input list and M is the length of the longest string in the input list. The sort() method used to sort the input list has a time complexity of O(NlogN), and the loop through the characters in the first string has a time complexity of O(M). The all() function inside the loop has a time complexity of O(N), where N is the number of strings in the input list. Therefore, the total time complexity is O(NlogN + NM).

Auxiliary space: O(1). The function uses a constant amount of auxiliary space, since it only stores a prefix string of length at most M.



Last Updated : 26 Apr, 2023
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