Find the Earliest Repeating Character

Last Updated : 07 Nov, 2024

Given a string S of length n, the task is to find the earliest repeated character in it. The earliest repeated character means, the character that occurs more than once and whose second occurrence has the smallest index.

Example:

Input: s = "geeksforgeeks"
Output: e
Explanation: e is the first element that repeats
Input: s = "hello geeks"
Output: l
Explanation: l is the first element that repeats

Table of Content

[Naive Approach] Using Two Nested Loops - O(n²) time and O(1) auxiliary space

The simplest way to solve this problem is to use a simple nested loop. The key idea here is to iterate through each character in the string and for each character, check if it has already appeared earlier in the string. If a character is found to be repeating, returned the currently picked character immediately as the result. If no character repeats, the returns "-1".

C++

#include <bits/stdc++.h>
using namespace std;

string firstRepChar(string& s)
{
    // Get the size of the input string
    int n = s.size();

    // Iterate through each character in the string
    for (int i = 0; i < n; i++) {

        // Check if the current character is a repeating
        // character
        for (int j = 0; j < i; j++) {
            if (s[i] == s[j]) {

                // Create a string to hold the repeating
                // character
                string result = "";
                result += s[i];
                return result;
            }
        }
    }

    // If no repeating character is found, return "-1"
    return "-1";
}

// Driver code
int main()
{
    string s = "geeksforgeeks";
    cout << firstRepChar(s);
    return 0;
}

Java

import java.util.Scanner;

public class FirstRepeatingCharacter {

    // Function to find the first repeating character in the
    // string
    public static String firstRepChar(String s)
    {
        // Get the size of the input string
        int n = s.length();

        // Iterate through each character in the string
        for (int i = 0; i < n; i++) {

            // Check if the current character is a repeating
            // character
            for (int j = 0; j < i; j++) {
                if (s.charAt(i) == s.charAt(j)) {

                    // Create a string to hold the repeating
                    // character
                    return Character.toString(s.charAt(i));
                }
            }
        }

        // If no repeating character is found, return "-1"
        return "-1";
    }

    public static void main(String[] args)
    {
        // Example usage:
        String s = "geeksforgeeks";

        // Print the result of the function
        System.out.println(firstRepChar(s));
    }
}

Python

def firstRepChar(s):

    # Get the size of the input string
    n = len(s)

    # Iterate through each character in the string
    for i in range(n):

        # Check if the current character is a repeating character
        for j in range(i):
            if s[i] == s[j]:

                # Return the repeating character
                return s[i]

    # If no repeating character is found, return "-1"
    return "-1"


# Example usage:
s = "geeksforgeeks"
print(firstRepChar(s))

using System;

public class Program {
    // Function to find the first repeating character in the
    // string
    public static string FirstRepChar(string s)
    {
        // Get the size of the input string
        int n = s.Length;

        // Iterate through each character in the string
        for (int i = 0; i < n; i++) {
            // Check if the current character is a repeating
            // character
            for (int j = 0; j < i; j++) {
                if (s[i] == s[j]) {
                    // Return the repeating character
                    return s[i].ToString();
                }
            }
        }

        // If no repeating character is found, return "-1"
        return "-1";
    }

    public static void Main()
    {
        // Example usage:
        string s = "geeksforgeeks";

        // Print the result of the function
        Console.WriteLine(FirstRepChar(s));
    }
}

JavaScript

function firstRepChar(s) {
    // Get the size of the input string
    let n = s.length;

    // Iterate through each character in the string
    for (let i = 0; i < n; i++) {
    
        // Check if the current character
        // is a repeating character
        for (let j = 0; j < i; j++) {
            if (s[i] === s[j]) {
            
                // Return the repeating character
                return s[i];
            }
        }
    }

    // If no repeating character is found, return "-1"
    return "-1";
}

// Example usage:
let s = "geeksforgeeks";
console.log(firstRepChar(s));

Output

Time Complexity: O(n²)
Auxiliary Space: O(1)

[Expected Approach] Using Frequency Counting - O(n) time and O(1) auxiliary space

The main idea behind this approach is to efficiently track the frequency/occurrences of each character in the string using an array or hash table. Here we have used array for the same.
Here is a more detailed step-by-step explanation of the approach:
We create an array of size 26 to store the count of each character. The size 26 is chosen because there are 26 letters in the English alphabet. Each index of this array will correspond to a character from 'a' to 'z'. For example, the index 0 corresponds to 'a', index 1 corresponds to 'b', and so on.
We iterate through each character in the input string. For each character, we calculate its corresponding index in the array by subtracting the ASCII value of 'a' from the ASCII value of the character. This way, 'a' maps to index 0, 'b' maps to index 1, and so forth.
For each character in the string, we increment the value at its corresponding index in the array. If the value at this index is greater than 0, it means the character has appeared before, and we return it as the first repeated character.
If we traverse the entire string and do not find any repeated characters, we return "-1" to indicate that there are no repeated characters in the string.

C++

#include <bits/stdc++.h>
using namespace std;

// Function to find the first repeated character in a string
string firstRepChar(string& s)
{
    // Create an array to store the count of characters
    int charCount[26] = { 0 };

    // Iterate through each character in the string
    for (int i = 0; i < s.length(); i++) {
        char ch = s[i];

        // Calculate the index in the array for this
        // character
        int index = ch - 'a';

        // If the count of the character is not zero,
        // it means the character is repeated, so we return
        // it
        if (charCount[index] != 0)
            return string(1, ch);

        // Increment the count of the character in the array
        charCount[index]++;
    }

    // If no character is repeated, return "-1"
    return "-1";
}

// Driver Code
int main()
{
    string s = "geeksforgeeks";

    // Call the function to find the first repeated
    // character
    cout << firstRepChar(s);

    return 0;
}

Java

import java.util.Arrays;

public class FirstRepeatingCharacter {
    // Function to find the first repeated character in a
    // string
    public static String firstRepChar(String s)
    {
        // Create an array to store the count of characters
        int[] charCount = new int[26];

        // Iterate through each character in the string
        for (int i = 0; i < s.length(); i++) {
            char ch = s.charAt(i);

            // Calculate the index in the array for this
            // character
            int index = ch - 'a';

            // If the count of the character is not zero,
            // it means the character is repeated, so we
            // return it
            if (charCount[index] != 0) {
                return Character.toString(ch);
            }

            // Increment the count of the character in the
            // array
            charCount[index]++;
        }

        // If no character is repeated, return "-1"
        return "-1";
    }

    public static void main(String[] args)
    {
        // Example usage:
        String s = "geeksforgeeks";

        // Print the result of the function
        System.out.println(firstRepChar(s));
    }
}

Python

def firstRepChar(s):
    # Create an array to store the count of characters
    charCount = [0] * 26

    # Iterate through each character in the string
    for ch in s:

        # Calculate the index in the array for this character
        index = ord(ch) - ord('a')

        # If the count of the character is not zero,
        # it means the character is repeated, so we return it
        if charCount[index] != 0:
            return ch

        # Increment the count of the character in the array
        charCount[index] += 1

    # If no character is repeated, return "-1"
    return "-1"


# Example usage:
s = "geeksforgeeks"
print(firstRepChar(s))

using System;

public class Program {
    // Function to find the first repeated character in a
    // string
    public static string FirstRepChar(string s)
    {
        // Create an array to store the count of characters
        int[] charCount = new int[26];

        // Iterate through each character in the string
        foreach(char ch in s)
        {
            // Calculate the index in the array for this
            // character
            int index = ch - 'a';

            // If the count of the character is not zero,
            // it means the character is repeated, so we
            // return it
            if (charCount[index] != 0) {
                return ch.ToString();
            }

            // Increment the count of the character in the
            // array
            charCount[index]++;
        }

        // If no character is repeated, return "-1"
        return "-1";
    }

    public static void Main()
    {
        // Example usage:
        string s = "geeksforgeeks";

        // Print the result of the function
        Console.WriteLine(FirstRepChar(s));
    }
}

JavaScript

function firstRepChar(s) {
    // Create an array to store the count of characters
    let charCount = new Array(26).fill(0);

    // Iterate through each character in the string
    for (let i = 0; i < s.length; i++) {
        let ch = s[i];

        // Calculate the index in the array for this character
        let index = ch.charCodeAt(0) - 'a'.charCodeAt(0);

        // If the count of the character is not zero,
        // it means the character is repeated, so we return it
        if (charCount[index] !== 0) {
            return ch;
        }

        // Increment the count of the character in the array
        charCount[index]++;
    }

    // If no character is repeated, return "-1"
    return "-1";
}

// Example usage:
let s = "geeksforgeeks";
console.log(firstRepChar(s));

Output

Time Complexity: O(n)
Auxiliary Space: O(1)

Similar Problem:

Find the Earliest Repeating Character

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Practice Tags :

Find the Earliest Repeating Character

[Naive Approach] Using Two Nested Loops - O(n2) time and O(1) auxiliary space

[Expected Approach] Using Frequency Counting - O(n) time and O(1) auxiliary space

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[Naive Approach] Using Two Nested Loops - O(n²) time and O(1) auxiliary space