C#: Remove duplicate characters from a given string

Last update on August 25 2025 12:50:25 (UTC/GMT +8 hours)

Write a C# Sharp program to remove duplicate characters from a given string.

Sample Solution:-

C# Sharp Code:

using System;
using System.Collections.Generic;

namespace exercises
{
    class Program
    {
        static void Main(string[] args)
        {
            // Define and display original string
            String str1;
            str1 = "aaaaaabbbbccc";
            Console.WriteLine("Original String: " + str1);

            // Display the string after removing duplicate characters
            Console.WriteLine("After removing duplicates characters from the said string:");
            Console.WriteLine(remove_duplicate_chars(str1));

            // Repeat the process for different strings
            str1 = "Python";
            Console.WriteLine("Original String: " + str1);
            Console.WriteLine("After removing duplicates characters from the said string:");
            Console.WriteLine(remove_duplicate_chars(str1));

            str1 = "Java";
            Console.WriteLine("Original String: " + str1);
            Console.WriteLine("After removing duplicates characters from the said string:");
            Console.WriteLine(remove_duplicate_chars(str1));
        }

        // Method to remove duplicate characters from a string
        public static string remove_duplicate_chars(string str1)
        {
            var indexes = new Dictionary<char, int>();

            // Store the last index of each character
            for (int i = 0; i < str1.Length; i++)
                indexes[str1[i]] = i;

            var flag = new HashSet<char>();
            var stack = new Stack<char>();

            for (int i = 0; i < str1.Length; i++)
            {
                var ele = str1[i];
                if (!flag.Contains(ele))
                {
                    // Remove elements from the stack that are greater than current and occur later in the string
                    while (stack.Count > 0 && stack.Peek() > ele && i < indexes[stack.Peek()])
                        flag.Remove(stack.Pop());

                    flag.Add(ele);
                    stack.Push(ele);
                }
            }

            var s = new char[stack.Count];
            int index = stack.Count - 1;

            // Reverse the stack content to restore original order
            foreach (var ele in stack)
                s[index--] = ele;

            return new string(s);
        }
    }
}

Sample Output:

Original String: aaaaaabbbbccc
After removing duplicates characters from the said string:
abc
Original String: Python
After removing duplicates characters from the said string:
Python
Original String: Java
After removing duplicates characters from the said string:
Jav

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