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

• Difficulty Level : Medium
• Last Updated : 30 Apr, 2021

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```

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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``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 = "``<

## 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

 ` 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``?>`

## Javascript

 ``

Output:

`Char = a   and Index = 6`

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