First Repeating Character in C, C++, Java & Python – Code with Explanation & Examples in Short and Simple

   

C Program

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

int main() {
    char str[100];
    scanf("%s", str);
    int freq[256] = {0};
    for (int i = 0; str[i]; i++) {
        if (freq[(unsigned char)str[i]]) {
            printf("%c\n", str[i]);
            return 0;
        }
        freq[(unsigned char)str[i]]++;
    }
    printf("No Repeating Character\n");
    return 0;
}

C Output

Input:
programming
Output:
r

C++ Program

#include <iostream>
#include <string>
using namespace std;

int main() {
    string s;
    cin >> s;
    int freq[256] = {0};
    for (char c : s) {
        if (freq[(unsigned char)c]) {
            cout << c << endl;
            return 0;
        }
        freq[(unsigned char)c]++;
    }
    cout << "No Repeating Character\n";
}

C++ Output

Input:
hello
Output:
l

JAVA Program

import java.util.*;
class Main {
    public static void main(String[] args) {
        Scanner sc = new Scanner(System.in);
        String s = sc.next();
        int[] freq = new int[256];
        for (char c : s.toCharArray()) {
            if (freq[c] > 0) {
                System.out.println(c);
                return;
            }
            freq[c]++;
        }
        System.out.println("No Repeating Character");
    }
}

JAVA Output

Input:
success
Output:
s

Python Program

s = input()
seen = set()
for c in s:
    if c in seen:
        print(c)
        break
    seen.add(c)
else:
    print("No Repeating Character")

Python Output

Input:
level
Output:
l

In-Depth Explanation

Example

When the input is "success", we begin scanning each character. The first s comes up, we tag it as seen. Then u, then c. When we see c again, we realize this is the first repeating character, therefore the output is c. It doesn't matter that latter s also repeats, since c repeats sooner in sequence.

Real-Life Analogy

Consider a line of individuals entering a room. You take note of each person's name as he or she enters. When a person enters again and you have already noted their name, that individual is the "first repeater." That is precisely what the program does with characters. Similar to an attendance system, where the first repeated entry means somebody has registered twice, our reasoning marks the first repeating character.

Why It Matters

Repeating characters are not merely an oddity in strings—they are significant in data validation, error detection, and pattern matching. For instance, in password systems, identifying repeating patterns can enhance security regulations. In compression algorithms for text, repeating symbols are involved in minimizing storage space. So that this very basic idea gets so influential in actual computing.

Learning Insights

This exercise educates beginners on how to employ arrays, sets, and hashing to address actual issues. It points out the significance of time complexity. Without hashing, one could compare each character to all others, which is O(n²). But with hashing or sets, it is O(n), which is far quicker. This illustrates how data structures enhance efficiency.

Interview Perspective

Interviewers adore such problems since they not only test coding, but also thought. A brute force solution might be okay, but an optimization using a hash table, set, or frequency array demonstrates more wisdom. It also tests if the candidate has string manipulation and data handling efficiency down pat. Another favorite follow-up question is to ask for the "first non-repeating character," which is more difficult and tests higher-level thinking.

Real-World Uses

In actual situations, finding the first recurring event is important. Think about a bank finding duplicate transactions or an email server finding spam patterns. Even in ordinary log file analysis, you might be interested in knowing the first recurring error message. This makes the problem immediately relevant to real-world coding.

SEO-Optimized Closing Explanation

Learning about finding the first repeating character in a string is an essential programming concept. It not only improves knowledge of arrays, hash tables, and sets but also teaches how to optimize from O(n²) brute force to O(n) efficient solutions. Whether you’re preparing for coding interviews, learning data structures, or solving competitive programming challenges, this problem strengthens logical thinking and string handling skills. By solving string problems such as palindrome validation, anagram identification, and the first repeating character, beginners and students build confidence in actual problem solving and in interview preparation.