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
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 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 <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; } |
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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 |
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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 = 26Str = "aabc"firstnonrepeating(Str) # This code is contributed by PranchalK |
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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 |
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Output:
a -1 b b
Video:
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