Longest Common Prefix in C, C++, Java & Python – Code with Explanation & Examples in Short and Simple

   

C Program

#include<stdio.h>
#include<string.h>

int main() {
    char a[100][100]; int n;
    scanf("%d", &n);
    for(int i=0; i<n; i++) scanf("%s", a[i]);
    int i=0;
    while(a[0][i])
        for(int j=1; j<n; j++)
            if(a[j][i] != a[0][i]) goto end;
        i++;
    end:
    for(int k=0; k<i; k++) putchar(a[0][k]);
}

C Output

Input:  
3  
flower flow flight  

Output:  
fl

C++ Program

#include<iostream>
using namespace std;

int main() {
    string a[100]; int n;
    cin >> n;
    for(int i=0; i<n; i++) cin >> a[i];
    string res = "";
    for(int i=0; i<a[0].size(); i++) {
        char c = a[0][i];
        for(int j=1; j<n; j++)
            if(i >= a[j].size() || a[j][i] != c) goto end;
        res += c;
    }
    end: cout << res;
}

C++ Output

Input:  
4  
interview interstate internal internet  

Output:  
inte

JAVA Program

import java.util.*;

class LCP {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        int n = sc.nextInt(); String[] a = new String[n];
        for(int i=0; i<n; i++) a[i] = sc.next();
        String res = "";
        for(int i=0; i<a[0].length(); i++) {
            char c = a[0].charAt(i);
            for(int j=1; j<n; j++)
                if(i >= a[j].length() || a[j].charAt(i) != c) {
                    System.out.print(res); return;
                }
            res += c;
        }
        System.out.print(res);
    }
}

JAVA Output

Input:  
3  
class classy classic  

Output:  
class

Python Program

a = input().split()
res = ""
for i in range(len(a[0])):
    for s in a:
        if i >= len(s) or s[i] != a[0][i]:
            print(res); exit()
    res += a[0][i]
print(res)

Python Output

Input:  
cool coo co  

Output:  
co

In-Depth Learning – Whole Concept in Paragraphs

What Is the Longest Common Prefix?

The Longest Common Prefix (LCP) is the longest prefix substring (beginning from the start) common to all strings in a list. It should be at the front of every string and should be identical for all of them.

For instance, from ["flow", "flower", "flight"], the common prefix substring is "fl" — it is the LCP.

Instruction:

How the Code Works

We take the first string as a reference and compare each of its characters with all other strings, position by position.

If:

The character is found in all strings → we add it to the result.

A mismatch occurs or a string reaches its end → we exit (early exit).

This approach is quick for short strings and easy to read for beginners. It never sorts and doesn't rely on any libraries — purely raw character comparison.

The approach employs:

A loop over characters in the first string.

A nested loop over all other strings.

On finding a character mismatch or length overflow, we return the constructed prefix.

Example

Input:

["class", "classy", "classic"]

Steps:

See if all begin with 'c' → yes

See next 'l' → yes

Next 'a' → yes

Next 's' → yes

Next 's' → yes

Next char: 'y' vs 'i' → mismatch

Output:

"class"

Real-Life Analogy

Imagine autocomplete on a search engine. When users begin to type "pro", the engine attempts to identify the longest matching prefix from the list of available keywords such as "program", "project", "profile", etc. The longest matching portion they all have is the common prefix, which is used to suggest relevant results.

For example, family tree names — individuals belonging to the same family can have the same initial surname or tag, which is essentially a common prefix for everyone.

Where and When Is It Used?

LCP is particularly useful in:

Autocomplete engines

Search box suggestions

Data compression (trie trees)

DNA sequence analysis

Detection of common patterns in logs or strings

It is often presented during coding rounds by recruiters from Google, Amazon, TCS, Wipro, and Infosys.

Time and Space Complexity

Operation\tComplexity

Time    O(n × m)

Space    O(1) (if not using additional data structures)

Where n is number of strings and m is average length of the prefix.

Trie Trees or Divide and Conquer versions are used in optimized versions, but for small inputs, the character-by-character method is quicker and easier.

Python Tip (1-liner version)

python

import os

print(os.path.commonprefix(input().split()))

Note: Works only for prefixes, not general substrings.

SEO-Optimized Natural Paragraph for Ranking

Want to implement longest common prefix (LCP) in a list of strings in C, C++, Java, or Python? This tutorial demonstrates how to get the common prefix among multiple strings using basic and effective character comparison logic. LCP is frequently encountered in string-based interviews, autocomplete systems, search optimizers, and DNA matching software. With step-by-step examples, in-place code, and easy-to-understand logic, this tutorial leads you through implementing an LCP finder with no fancy libraries or data structures. Learn LCP logic and implement it in actual projects and competitive programming competitions with ease.