C++ Program To Find Longest Common Prefix Using Sorting

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"

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++ 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

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. 
Auxiliary Space: O(1) as no extra space has been used.

Please refer complete article on Longest Common Prefix using Sorting for more details!

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Article Tags :

• Strings
• Sorting
• C++ Programs
• DSA
• Longest Common Prefix

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Practice Tags :

• Sorting
• Strings