Find first non-repeating character in a given string using Linked List

Given a string str of length L, the task is to find the first occurrence of a non-repeating character in the string.
Examples:

Input: str = “geeksforgeeks”
Output: f

Input: str = “programmer”
Output: p

Note: Please refer this article for HashMap approach and this article as space-optimized approach.

Linked List Approach:  The idea is to use Linked List to keep track of the unique elements in the string. Below is the illustration of the approach:



  • Iterate over the string for each character in the string and add the character in the Linked List on basis of the below conditions:
    • If the character is already present in the Linked List, then remove the existing character node from the linked list.
    • Otherwise, Add the character into the linked list.
  • Finally, the character at the first node of the linked list is the first non-repeating character of the string.

Below is the implementation of the above approach:

Java

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// Java implementation to find the 
// first non-repeating element 
// of the string using Linked List 
  
import java.util.LinkedList; 
  
public class FirstNonRepeatingElement { 
  
    // Function to find the first 
    // non-repeating element of the 
    // given string using Linked List 
    static void firstNonRepElement(String str) 
    
        LinkedList<Character> list 
            = new LinkedList<Character>(); 
  
        list.add(str.charAt(0)); 
  
        for (int i = 1; i < str.length(); i++) { 
            if (list.contains(str.charAt(i))) 
                list.remove(list.indexOf( 
                    str.charAt(i))); 
            else
                list.add(str.charAt(i)); 
        
        System.out.println(list.get(0)); 
    
  
    // Driver Code 
    public static void main(String[] args) 
    
        String str = "geeksforgeeks"
  
        // Function Call 
        firstNonRepElement(str); 
    
}

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C#

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// C# implementation to find the
// first non-repeating element
// of the string using Linked List
using System; 
using System.Collections; 
using System.Collections.Generic;
  
class FirstNonRepeatingElement{
  
// Function to find the first
// non-repeating element of the
// given string using Linked List
static void firstNonRepElement(string str)
{
    LinkedList<char> list = new LinkedList<char>();
  
    list.AddLast(str[0]);
  
    for(int i = 1; i < str.Length; i++) 
    {
        if (list.Contains(str[i]))
            list.Remove(str[i]);
        else
            list.AddLast(str[i]);
    }
    Console.Write(list.First.Value);
}
  
// Driver Code
public static void Main(string[] args)
{
    string str = "geeksforgeeks";
  
    // Function call
    firstNonRepElement(str);
}
}
  
// This code is contributed by rutvik_56

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Output

f

Performance Analysis:

  • Time Complexity: O(N * 26)
  • Auxiliary Space: O(N)

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Improved By : rutvik_56