# Queue based approach for first non-repeating character in a stream

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.

## C++

```// C++ program for a Queue based approach to find first non-repeating
// character
#include <bits/stdc++.h>
using namespace std;
const int CHAR_MAX = 26;

// function to find first non repeating
// character of sa stream
void firstnonrepeating(char str[])
{
queue<char> q;
int charCount[CHAR_MAX] = { 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.Queue;

public class NonReapatingCQueue
{
final static int CHAR_MAX = 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[CHAR_MAX];

//Queue to store Characters

// traverse whole stream
for(int i=0; i<str.length(); i++)
{
char c = str.charAt(i);

// push each character in queue

// increment the frequency count
charCount1++;

// 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
```

Output:

```a -1 b b
```

This article is contributed by Niteesh Kumar. If you like GeeksforGeeks and would like to contribute, you can also write an article using contribute.geeksforgeeks.org or mail your article to contribute@geeksforgeeks.org. See your article appearing on the GeeksforGeeks main page and help other Geeks.

# GATE CS Corner    Company Wise Coding Practice

Please write to us at contribute@geeksforgeeks.org to report any issue with the above content.
2.9 Average Difficulty : 2.9/5.0
Based on 13 vote(s)