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

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 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 using namespace std; const int MAX_CHAR = 26;   // function to find first non repeating // character of sa stream void firstnonrepeating(char str[]) {     queue 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 =  * 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) -                           ord('a')] > 1):                 q.get()             else:                 print(q.queue, 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 q = new LinkedList();           // 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



Output:

a -1 b b

Video:

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