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

Difficulty Level : Medium
Last Updated : 22 Jun, 2021

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

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

Create a count array of size 26(assuming only lower case characters are present) and initialize it with zero.
Create a queue of char datatype.
Store each character in queue and increase its frequency in the hash array.
For every character of stream, we check front of the queue.
If the frequency of character at the front of queue is one, then that will be the first non-repeating character.
Else if frequency is more than 1, then we pop that element.
If queue became empty that means there are no non-repeating characters 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 <bits/stdc++.h>
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<Character> q = new LinkedList<Character>();
 
        // 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

Javascript

<script>
 
      // JavaScript Program for a Queue based approach
      // to find first non-repeating character
 
      const MAX_CHAR = 26;
 
      // function to find first non repeating
      // character of stream
      function firstNonRepeating(str) {
        // count array of size 26(assuming
        // only lower case characters are present)
        var charCount = new Array(MAX_CHAR).fill(0);
 
        // Queue to store Characters
        var q = [];
 
        // traverse whole stream
        for (var i = 0; i < str.length; i++) {
          var ch = str[i];
 
          // push each character in queue
          q.push(ch);
 
          // increment the frequency count
          charCount[ch.charCodeAt(0) - "a".charCodeAt(0)]++;
 
          // check for the non repeating character
          while (q.length > 0) {
            if (charCount[q[0].charCodeAt(0) -
            "a".charCodeAt(0)] > 1) {
              q.shift();
            } else {
              document.write(q[0] + " ");
              break;
            }
          }
          if (q.length == 0) {
            document.write(-1 + " ");
          }
        }
        document.write("<br>");
      }
 
      // Driver function
      var str = "aabc";
      firstNonRepeating(str);
       
</script>

Output:

a -1 b b

Video:

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