Efficiently find first repeated character in a string without using any additional data structure in one traversal

Implement a space efficient algorithm to check First repeated character in a string without using any additional data structure in one traversal. Use additional data structures like count array, hash, etc is not allowed.

Examples :

Input :  abcfdeacf
Output : char = a, index= 6

Recommended: Please solve it on “PRACTICE ” first, before moving on to the solution.

The idea is to use an integer variable and uses bits in its binary representation to store whether a character is present or not. Typically an integer has at-least 32 bits and we need to store presence/absence of only 26 characters.

C++

// Efficiently check First repeated character 
// in C++ program  
#include<bits/stdc++.h> 
using namespace std; 
  
// Returns -1 if all characters of str are 
// unique. 
// Assumptions : (1) str contains only characters 
//                 from 'a' to 'z' 
//             (2) integers are stored using 32 
//                 bits 
int FirstRepeated(string str) 
{ 
    // An integer to store presence/absence 
    // of 26 characters using its 32 bits. 
    int checker = 0; 
  
    for (int i = 0; i < str.length(); ++i) 
    { 
        int val = (str[i]-'a'); 
  
        // If bit corresponding to current 
        // character is already set 
        if ((checker & (1 << val)) > 0) 
            return i; 
  
        // set bit in checker 
        checker |= (1 << val); 
    } 
  
    return -1; 
} 
  
// Driver code 
int main() 
{ 
    string s = "abcfdeacf"; 
    int i=FirstRepeated(s); 
    if (i!=-1) 
        cout <<"Char = "<< s[i] << "   and Index = "<<i; 
    else
        cout << "No repeated Char"; 
    return 0; 
} 

Java

// Efficiently check First repeated character 
// in Java program  
public class First_Repeated_char { 
  
    static int FirstRepeated(String str) 
    { 
        // An integer to store presence/absence 
        // of 26 characters using its 32 bits. 
        int checker = 0; 
       
        for (int i = 0; i < str.length(); ++i) 
        { 
            int val = (str.charAt(i)-'a'); 
       
            // If bit corresponding to current 
            // character is already set 
            if ((checker & (1 << val)) > 0) 
                return i; 
       
            // set bit in checker 
            checker |= (1 << val); 
        } 
       
        return -1; 
    } 
       
    // Driver code 
    public static void main(String args[]) 
    { 
        String s = "abcfdeacf"; 
        int i=FirstRepeated(s); 
        if (i!=-1) 
           System.out.println("Char = "+ s.charAt(i) + "   and Index = "+i); 
        else
            System.out.println( "No repeated Char"); 
    } 
} 
// This code is contributed by Sumit Ghosh 

Python

# Efficiently check First repeated character 
# in Python  
  
# Returns -1 if all characters of str are 
# unique. 
# Assumptions : (1) str contains only characters 
#                 from 'a' to 'z' 
##             (2) integers are stored using 32 
##                 bits 
def FirstRepeated(string): 
      
    # An integer to store presence/absence 
    # of 26 characters using its 32 bits. 
    checker = 0
   
    pos = 0
    for i in string: 
        val = ord(i) - ord('a'); 
   
        # If bit corresponding to current 
        # character is already set 
        if ((checker & (1 << val)) > 0): 
            return pos 
   
        # set bit in checker 
        checker |= (1 << val) 
        pos += 1
   
    return -1
   
# Driver code 
string = "abcfdeacf"
i = FirstRepeated(string) 
if i != -1: 
    print "Char = ", string[i], " and Index = ", i; 
else: 
    print "No repeated Char"
  
# This code is contributed by Sachin Bisht 

C#

// C# program to Efficiently  
// check First repeated character 
using System; 
  
public class First_Repeated_char { 
  
    static int FirstRepeated(string str) 
    { 
        // An integer to store presence/absence 
        // of 26 characters using its 32 bits. 
        int checker = 0; 
      
        for (int i = 0; i < str.Length; ++i) 
        { 
            int val = (str[i]-'a'); 
      
            // If bit corresponding to current 
            // character is already set 
            if ((checker & (1 << val)) > 0) 
                return i; 
      
            // set bit in checker 
            checker |= (1 << val); 
        } 
      
        return -1; 
    } 
      
    // Driver code 
    public static void Main() 
    { 
        string s = "abcfdeacf"; 
        int i=FirstRepeated(s); 
        if (i!=-1) 
           Console.WriteLine("Char = " + s[i] + 
                          " and Index = " + i); 
        else
            Console.WriteLine( "No repeated Char"); 
    } 
} 
  
// This code is contributed by vt_m. 

PHP

<?php  
// Efficiently check First repeated character 
// in PHP program  
  
// Returns -1 if all characters of str are 
// unique. 
// Assumptions : (1) str contains only characters 
//                   from 'a' to 'z' 
//               (2) integers are stored using 32 
//                   bits 
function FirstRepeated($str) 
{ 
    // An integer to store presence/absence 
    // of 26 characters using its 32 bits. 
    $checker = 0; 
  
    for ($i = 0; $i < strlen($str); ++$i) 
    { 
        $val = (ord($str[$i]) - ord('a')); 
  
        // If bit corresponding to current 
        // character is already set 
        if (($checker & (1 << $val)) > 0) 
            return $i; 
  
        // set bit in checker 
        $checker |= (1 << $val); 
    } 
  
    return -1; 
} 
  
// Driver code 
$s = "abcfdeacf"; 
$i=FirstRepeated($s); 
if ($i!=-1) 
    echo "Char = " . $s[$i] . 
         " and Index = " . $i; 
else
    echo "No repeated Char"; 
  
// This code is contributed by ita_c 
?> 

Output:

Char = a   and Index = 6

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

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