Longest Common Prefix using Binary Search Last Updated : 23 Jul, 2025 Comments Improve Suggest changes Like Article Like Report Given an array of strings arr[], the task is to return the longest common prefix among each and every strings present in the array. If there’s no prefix common in all the strings, return "".Examples:Input: arr[] = [“geeksforgeeks”, “geeks”, “geek”, “geezer”]Output: "gee"Explanation: "gee" is the longest common prefix in all the given strings: "geeksforgeeks", "geeks", "geeks" and "geezer".Input: arr[] = ["apple", "ape", "april"]Output : "ap"Explanation: "ap" is the longest common prefix in all the given strings: "apple", "ape" and "april".Input: arr[] = ["hello", "world"]Output: ""Explanation: There’s no common prefix in the given strings.Approach:The idea is to use binary search on the indices to find the length of longest common prefix. We can observe that the length of common prefix cannot be greater than the length of smallest string in given set of strings. So the search space for binary search will be from 0 to length of smallest string - 1.In each iteration of binary search [low to high], we will check whether the prefix [low to mid] is common among all strings or not.If the prefix is common among all string then check for larger length prefix by updating low = mid + 1Else check for smaller length prefix, by updating high = mid - 1.Note: Instead of checking each character from 0 to mid, we will to check only substring from low to mid because substring from 0 to low would have been checked in previous iterations and found to be common. C++ // C++ program to find the longest common prefix // using Binary Search #include <iostream> #include <vector> using namespace std; // Function to check if the substring between the // indexes[left ... right] was common among all strings or not bool allContainSubstring(vector<string>& arr, int left, int right) { for (int i = 1; i <= arr.size() - 1; i++) { for (int j = left; j <= right; j++) { if (arr[i][j] != arr[0][j]) return false; } } return true; } // A Function that returns the longest common prefix // from the array of strings string longestCommonPrefix(vector<string>& arr) { int minLen = arr[0].length(); // Find length of shortest string for (int i = 1; i < arr.size(); i++) minLen = min(minLen, (int) arr[i].length()); // We will do an in-place binary search on the indices // to find the length of longest common prefix int lo = 0, hi = minLen - 1; int prefixLen = 0; while (lo <= hi) { int mid = lo + (hi - lo) / 2; if (allContainSubstring(arr, lo, mid)) { // If all the strings in the input array // contains common prefix till mid index // then update prefixLen with mid + 1 prefixLen = mid + 1; // And then check in the right part lo = mid + 1; } // Otherwise check in the left part else hi = mid - 1; } // return the common prefix by taking // substring from first string return arr[0].substr(0, prefixLen); } int main() { vector<string> arr = {"geeksforgeeks", "geeks", "geek", "geezer"}; cout << longestCommonPrefix(arr) << endl; return 0; } Java // Java program to find the longest common prefix // using Binary Search import java.util.List; class GfG { // Function to check if the substring between the // indexes[left ... right] was common among all strings or not static boolean allContainSubstring(String[] arr, int left, int right) { for (int i = 1; i < arr.length; i++) { for (int j = left; j <= right; j++) { if (arr[i].charAt(j) != arr[0].charAt(j)) { return false; } } } return true; } // A Function that returns the longest common prefix // from the array of strings static String longestCommonPrefix(String []arr) { int minLen = arr[0].length(); // Find length of shortest string for (String str : arr) { minLen = Math.min(minLen, str.length()); } // We will do an in-place binary search on the indices // to find the length of longest common prefix int lo = 0, hi = minLen - 1; int prefixLen = 0; while (lo <= hi) { int mid = lo + (hi - lo) / 2; if (allContainSubstring(arr, lo, mid)) { // If all the strings in the input array // contain common prefix till mid index // then update prefixLen with mid + 1 prefixLen = mid + 1; // And then check in the right part lo = mid + 1; } else { // Otherwise check in the left part hi = mid - 1; } } // Return the common prefix by taking // substring from the first string return arr[0].substring(0, prefixLen); } public static void main(String[] args) { String[] arr = {"geeksforgeeks", "geeks", "geek", "geezer"}; System.out.println(longestCommonPrefix(arr)); } } Python # Python program to find the longest common prefix # using Binary Search # Function to check if the substring between the # indexes[left ... right] was common among all strings or not def allContainSubstring(arr, left, right): for i in range(1, len(arr)): for j in range(left, right + 1): if arr[i][j] != arr[0][j]: return False return True # A Function that returns the longest common prefix # from the array of strings def longestCommonPrefix(arr): minLen = len(arr[0]) # Find length of shortest string for s in arr: minLen = min(minLen, len(s)) # We will do an in-place binary search on the indices # to find the length of longest common prefix lo, hi = 0, minLen - 1 prefixLen = 0 while lo <= hi: mid = lo + (hi - lo) // 2 if allContainSubstring(arr, lo, mid): # If all the strings in the input array # contain common prefix till mid index # then update prefixLen with mid + 1 prefixLen = mid + 1 # And then check in the right part lo = mid + 1 else: # Otherwise check in the left part hi = mid - 1 # Return the common prefix by taking # substring from the first string return arr[0][:prefixLen] if __name__ == "__main__": arr = ["geeksforgeeks", "geeks", "geek", "geezer"] print(longestCommonPrefix(arr)) C# // C# program to find the longest common prefix // using Binary Search using System; using System.Collections.Generic; class GfG { // Function to check if the substring between the // indexes [left ... right] was common among all strings or not static bool allContainSubstring(string[] arr, int left, int right) { for (int i = 1; i < arr.Length; i++) { for (int j = left; j <= right; j++) { if (arr[i][j] != arr[0][j]) return false; } } return true; } // A Function that returns the longest common prefix // from the array of strings static string longestCommonPrefix(string[] arr) { int minLen = arr[0].Length; // Find length of shortest string foreach (string str in arr) { minLen = Math.Min(minLen, str.Length); } // We will do an in-place binary search on the indices // to find the length of longest common prefix int lo = 0, hi = minLen - 1; int prefixLen = 0; while (lo <= hi) { int mid = lo + (hi - lo) / 2; if (allContainSubstring(arr, lo, mid)) { // If all the strings in the input array // contain common prefix till mid index // then update prefixLen with mid + 1 prefixLen = mid + 1; // And then check in the right part lo = mid + 1; } else { // Otherwise check in the left part hi = mid - 1; } } // Return the common prefix by taking // substring from the first string return arr[0].Substring(0, prefixLen); } static void Main() { string[] arr = {"geeksforgeeks", "geeks", "geek", "geezer"}; Console.WriteLine(longestCommonPrefix(arr)); } } JavaScript // JavaScript program to find the longest common prefix // using Binary Search // Function to check if the substring between the // indexes [left ... right] was common among all strings or not function allContainSubstring(arr, left, right) { for (let i = 1; i < arr.length; i++) { for (let j = left; j <= right; j++) { if (arr[i][j] !== arr[0][j]) return false; } } return true; } // A Function that returns the longest common prefix // from the array of strings function longestCommonPrefix(arr) { let minLen = arr[0].length; // Find length of shortest string for (let str of arr) { minLen = Math.min(minLen, str.length); } // We will do an in-place binary search on the indices // to find the length of longest common prefix let lo = 0, hi = minLen - 1; let prefixLen = 0; while (lo <= hi) { const mid = lo + Math.floor((hi - lo) / 2); if (allContainSubstring(arr, lo, mid)) { // If all the strings in the input array // contain common prefix till mid index // then update prefixLen with mid + 1 prefixLen = mid + 1; // And then check in the right part lo = mid + 1; } else { // Otherwise check in the left part hi = mid - 1; } } // Return the common prefix by taking // substring from the first string return arr[0].substring(0, prefixLen); } // Driver Code const arr = ["geeksforgeeks", "geeks", "geek", "geezer"]; console.log(longestCommonPrefix(arr)); Outputgee Time Complexity : O(n*m), where n is number of strings and m is length of shortest string. The recurrence relation is: T(m) = T(m/2) + O(n*(m/2)). Upon solving, we get time complexity of O(n*m).Auxiliary Space: O(m), to store the longest prefix string.Other Approaches:Longest Common Prefix using SortingLongest Common Prefix Word by Word MatchingLongest Common Prefix Character by Character MatchingLongest Common Prefix Divide and ConquerLongest Common Prefix Using Trie Comment More infoAdvertise with us Next Article Analysis of Algorithms K kartik Improve Article Tags : Strings DSA Amazon Accolite MakeMyTrip Binary Search Longest Common Prefix strings +4 More Practice Tags : AccoliteAmazonMakeMyTripBinary SearchStringsStrings +2 More Similar Reads Basics & PrerequisitesLogic Building ProblemsLogic building is about creating clear, step-by-step methods to solve problems using simple rules and principles. 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