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.
Below is the implementation of above approach:
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 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.push_back(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-1; 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 |
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 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)
This article is contributed by Rachit Belwariar. If you like GeeksforGeeks and would like to contribute, you can also write an article and mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.
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