Find the first repeated character in a string

Given a string, find the first repeated character in it. We need to find the character that occurs more than once and whose index of second occurrence is smallest. A variation of this question is discussed here.

Examples:

Input: ch = “geeksforgeeks” 
Output: e 
e is the first element that repeats

Input: str = “hello geeks” 
Output: l 
l is the first element that repeats

Simple Solution: The solution is to run two nested loops. Start traversing from left side. For every character, check if it repeats or not. If the character repeats, increment count of repeating characters. When the count becomes K, return the character. 
Time Complexity of this solution is O(n²)

We can Use Sorting to solve the problem in O(n Log n) time. Following are detailed steps.  

• Copy the given array to an auxiliary array temp[].
• Sort the temp array using a O(N log N) time sorting algorithm.
• Scan the input array from left to right. For every element, count its occurrences in temp[] using binary search. As soon as we find a character that occurs more than once, we return the character.

This step can be done in O(N Log N) time.

An efficient solution is to use Hashing to solve this in O(N) time on average. 

• Create an empty hash.
• Scan each character of input string and insert values to each keys in the hash.
• When any character appears more than once, hash key value is increment by 1, and return the character.

Below image is a dry run of the above approach:

Below is the implementation of the above approach:

C++

// CPP program to find the first
// repeated character in a string
#include <bits/stdc++.h>
using namespace std;

// Returns first repeating character in str.
char firstRepeating(string &str)
{
    // Creates an empty hashset
    unordered_set<char> h;

    // Traverse the input array from left to right
    for (int i=0; i<str.length(); i++)
    {
        char c = str[i];

        // If element is already in hash set, update x
        // and then break
        if (h.find(c) != h.end())
            return c;

        else // Else add element to hash set
            h.insert(c);
    }

    // If there was no repeated character
    return '\0';
}

// Driver method to test above method
int main ()
{
    string str = "geeksforgeeks";
    cout << firstRepeating(str);
    return 0;
}

Java

// Java program to find the first
// repeated character in a string
import java.util.*;

class Main
{
    // This function prints the first repeated
    // character in str[]
    static char firstRepeating(char str[])
    {
        // Creates an empty hashset
        HashSet<Character> h = new HashSet<>();

        // Traverse the input array from left to right
        for (int i=0; i<=str.length-1; i++)
        {
            char c = str[i];

            // If element is already in hash set, update x
            // and then break
            if (h.contains(c))
                return c;

            else // Else add element to hash set
                h.add(c);
        }

        return '\0';
    }

    // Driver method to test above method
    public static void main (String[] args)
    {
        String str = "geeksforgeeks";
        char[] arr = str.toCharArray();
        System.out.println(firstRepeating(arr));
    }
}

Python3

# Python program to find the first
# repeated character in a string
def firstRepeatedChar(str):

    h = {} # Create empty hash

    # Traverse each characters in string
    # in lower case order
    for ch in str:

        # If character is already present
        # in hash, return char
        if ch in h:
            return ch;

        # Add ch to hash
        else:
            h[ch] = 0

    return ''


# Driver code
print(firstRepeatedChar("geeksforgeeks"))

C#

// C# program to find the first
// repeated character in a string
using System;
using System.Collections.Generic;

class GFG
{
// This function prints the first
// repeated character in str[]
public static char firstRepeating(char[] str)
{
    // Creates an empty hashset
    HashSet<char> h = new HashSet<char>();

    // Traverse the input array
    // from left to right
    for (int i = 0; i <= str.Length - 1; i++)
    {
        char c = str[i];

        // If element is already in hash set,
        // update x and then break
        if (h.Contains(c))
        {
            return c;
        }

        else // Else add element to hash set
        {
            h.Add(c);
        }
    }

    return '\0';
}

// Driver Code
public static void Main(string[] args)
{
    string str = "geeksforgeeks";
    char[] arr = str.ToCharArray();
    Console.WriteLine(firstRepeating(arr));
}
}

// This code is contributed by Shrikant13

PHP

<?php
// PHP program to find the first repeated
// character in a string

// Returns first repeating character in str.
function firstRepeating($str)
{
    // Creates an empty hashset
    $h = array();

    // Traverse the input array
    // from left to right
    for ($i = 0; $i < strlen($str); $i++)
    {
        $c = $str[$i];

        // If element is already in hash
        // set, update x and then break
        if (array_search($c, $h))
            return $c;

        else // Else add element to hash set
            array_push($h, $c);
    }

    // If there was no repeated character
    return '\0';
}

// Driver Code
$str = "geeksforgeeks";
echo firstRepeating($str);

// This code is contributed by ita_c
?>

Javascript

<script>

// Javascript program to find the first
// repeated character in a string

// This function prints the first repeated
// character in str[]
function firstRepeating(str)
{

    // Creates an empty hashset
    let h = new Set();

    // Traverse the input array from left to right
    for(let i = 0; i <= str.length - 1; i++)
    {
        let c = str[i];

        // If element is already in hash
        // set, update x and then break
        if (h.has(c))
            return c;

        // Else add element to hash set
        else
            h.add(c);
    }
    return '\0';
}

// Driver code
let str = "geeksforgeeks";
document.write(firstRepeating(str));

// This code is contributed by avanitrachhadiya2155

</script>

Output:

e

Time complexity : O(n) 
Auxiliary Space : O(n)

Similar Problem: finding first non-repeated character in a string.

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