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Queue based approach for first non-repeating character in a stream
• Difficulty Level : Medium
• Last Updated : 09 Aug, 2019

Given a stream of characters and we have to find first non repeating character each time a character is inserted to the stream.

Examples:

```Input  : a a b c
Output : a -1 b b

Input  : a a c
Output : a -1 c
```

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

We have already discussed a Doubly linked list based approach in the previous post.

Approach-

1. Create a count array of size 26(assuming only lower case characters are present) and initialize it with zero.
2. Create a queue of char datatype.
3. Store each character in queue and increase its frequency in the hash array.
4. For every character of stream, we check front of the queue.
5. If the frequency of character at the front of queue is one, then that will be the first non repeating character.
6. Else if frequency is more than 1, then we pop that element.
7. If queue became empty that means there are no non repeating character so we will print -1.

Below is the implementation of above approach:

## C++

 `// C++ program for a Queue based approach ``// to find first non-repeating character``#include ``using` `namespace` `std;``const` `int` `MAX_CHAR = 26;`` ` `// function to find first non repeating``// character of sa stream``void` `firstnonrepeating(``char` `str[])``{``    ``queue<``char``> q;``    ``int` `charCount[MAX_CHAR] = { 0 };`` ` `    ``// traverse whole stream``    ``for` `(``int` `i = 0; str[i]; i++) {`` ` `        ``// push each character in queue``        ``q.push(str[i]);`` ` `        ``// increment the frequency count``        ``charCount[str[i] - ``'a'``]++;`` ` `        ``// check for the non pepeating character``        ``while` `(!q.empty()) {``            ``if` `(charCount[q.front() - ``'a'``] > 1)``                ``q.pop();``            ``else` `{``                ``cout << q.front() << ``" "``;``                ``break``;``            ``}``        ``}`` ` `        ``if` `(q.empty())``            ``cout << -1 << ``" "``;``    ``}``    ``cout << endl;``}`` ` `// Driver function``int` `main()``{``    ``char` `str[] = ``"aabc"``;``    ``firstnonrepeating(str);``    ``return` `0;``}`

## Java

 `// Java Program for a Queue based approach ``// to find first non-repeating character`` ` `import` `java.util.LinkedList;``import` `java.util.Queue;`` ` `public` `class` `NonReapatingCQueue {``    ``final` `static` `int` `MAX_CHAR = ``26``;`` ` `    ``// function to find first non repeating``    ``// character of stream``    ``static` `void` `firstNonRepeating(String str)``    ``{``        ``// count array of size 26(assuming``        ``// only lower case characters are present)``        ``int``[] charCount = ``new` `int``[MAX_CHAR];`` ` `        ``// Queue to store Characters``        ``Queue q = ``new` `LinkedList();`` ` `        ``// traverse whole stream``        ``for` `(``int` `i = ``0``; i < str.length(); i++) {``            ``char` `ch = str.charAt(i);`` ` `            ``// push each character in queue``            ``q.add(ch);`` ` `            ``// increment the frequency count``            ``charCount[ch - ``'a'``]++;`` ` `            ``// check for the non repeating character``            ``while` `(!q.isEmpty()) {``                ``if` `(charCount[q.peek() - ``'a'``] > ``1``)``                    ``q.remove();``                ``else` `{``                    ``System.out.print(q.peek() + ``" "``);``                    ``break``;``                ``}``            ``}``            ``if` `(q.isEmpty())``                ``System.out.print(-``1` `+ ``" "``);``        ``}``        ``System.out.println();``    ``}`` ` `    ``// Driver function``    ``public` `static` `void` `main(String[] args)``    ``{``        ``String str = ``"aabc"``;``        ``firstNonRepeating(str);``    ``}``}``// This code is Contributed by Sumit Ghosh`

## Python3

 `# Python3 program for a Queue based approach ``# to find first non-repeating character ``from` `queue ``import` `Queue `` ` `# function to find first non ``# repeating character of sa Stream ``def` `firstnonrepeating(``Str``):``    ``global` `MAX_CHAR``    ``q ``=` `Queue()``    ``charCount ``=` `[``0``] ``*` `MAX_CHAR ``     ` `    ``# traverse whole Stream``    ``for` `i ``in` `range``(``len``(``Str``)):`` ` `        ``# push each character in queue ``        ``q.put(``Str``[i]) `` ` `        ``# increment the frequency count ``        ``charCount[``ord``(``Str``[i]) ``-` `                  ``ord``(``'a'``)] ``+``=` `1`` ` `        ``# check for the non pepeating``        ``# character ``        ``while` `(``not` `q.empty()): ``            ``if` `(charCount[``ord``(q.queue[``0``]) ``-` `                          ``ord``(``'a'``)] > ``1``): ``                ``q.get() ``            ``else``:``                ``print``(q.queue[``0``], end ``=` `" "``) ``                ``break`` ` `        ``if` `(q.empty()): ``            ``print``(``-``1``, end ``=` `" "``)``    ``print``()`` ` `# Driver Code``MAX_CHAR ``=` `26``Str` `=` `"aabc"``firstnonrepeating(``Str``)`` ` `# This code is contributed by PranchalK`

## C#

 `using` `System;``using` `System.Collections.Generic;`` ` `// C# Program for a Queue based approach  ``// to find first non-repeating character `` ` ` ` `public` `class` `NonReapatingCQueue``{``    ``public` `const` `int` `MAX_CHAR = 26;`` ` `    ``// function to find first non repeating ``    ``// character of stream ``    ``public` `static` `void` `firstNonRepeating(``string` `str)``    ``{``        ``// count array of size 26(assuming ``        ``// only lower case characters are present) ``        ``int``[] charCount = ``new` `int``[MAX_CHAR];`` ` `        ``// Queue to store Characters ``        ``LinkedList<``char``> q = ``new` `LinkedList<``char``>();`` ` `        ``// traverse whole stream ``        ``for` `(``int` `i = 0; i < str.Length; i++)``        ``{``            ``char` `ch = str[i];`` ` `            ``// push each character in queue ``            ``q.AddLast(ch);`` ` `            ``// increment the frequency count ``            ``charCount[ch - ``'a'``]++;`` ` `            ``// check for the non repeating character ``            ``while` `(q.Count > 0)``            ``{``                ``if` `(charCount[q.First.Value - ``'a'``] > 1)``                ``{``                    ``q.RemoveFirst();``                ``}``                ``else``                ``{``                    ``Console.Write(q.First.Value + ``" "``);``                    ``break``;``                ``}``            ``}``            ``if` `(q.Count == 0)``            ``{``                ``Console.Write(-1 + ``" "``);``            ``}``        ``}``        ``Console.WriteLine();``    ``}`` ` `    ``// Driver function ``    ``public` `static` `void` `Main(``string``[] args)``    ``{``        ``string` `str = ``"aabc"``;``        ``firstNonRepeating(str);``    ``}``}`` ` `//This code is contributed by Shrikant13`
Output:
```a -1 b b
```

Video:

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