Question

How to find the first repeated character in a string with C#

Answer 1

using System;
using System.Collections.Generic;

public static class StringUtils
{
    /// <summary>
    /// Finds the first repeated character in a string.
    ///
    /// Examples:
    /// "abca"         -> 'a'
    /// "abcbd"        -> 'b'
    /// "abcde"        -> null
    /// "abcdxypcbop"  -> 'c'
    ///
    /// Time Complexity: O(n)
    /// Space Complexity: O(n)
    /// </summary>
    /// <param name="input">The input string.</param>
    /// <returns>
    /// The first repeated character, or null if no repeated character exists.
    /// </returns>
    public static char? FindFirstRepeatedCharacter(string input)
    {
        if (string.IsNullOrEmpty(input)) {
            return null;
        }

        var seen = new HashSet<char>();

        foreach (char ch in input)
        {
            // HashSet.Add() returns false if the character
            // already exists in the set.
            if (!seen.Add(ch)) {
                return ch;
            }
        }

        // No repeated character found.
        return null;
    }

    public static void Main()
    {
        Console.WriteLine(FindFirstRepeatedCharacter("abca"));         
        Console.WriteLine(FindFirstRepeatedCharacter("abcbd"));        
        Console.WriteLine(FindFirstRepeatedCharacter("abcde"));        
        Console.WriteLine(FindFirstRepeatedCharacter("abcdxypcbop"));  
    }
}
  
  
/*
run:
  
a
b

c
  
*/

 

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Answer 2

using System;
using System.Collections.Generic;

public static class StringUtils
{
    /// <summary>
    /// Finds the first repeated character in a string.
    ///
    /// Examples:
    /// "abca"         -> 'a'
    /// "abcbd"        -> 'b'
    /// "abcde"        -> null
    /// "abcdxypcbop"  -> 'c'
    ///
    /// Time Complexity: O(n)
    /// Space Complexity: O(n)
    /// </summary>
    /// <param name="input">The input string.</param>
    /// <returns>
    /// The first repeated character, or null if no repeated character exists.
    /// </returns>
    public static char? FindFirstRepeatedCharacter(string input)
    {
        if (string.IsNullOrEmpty(input)) {
            return null;
        }

        var seen = new HashSet<char>();

        foreach (char ch in input)
        {
            // HashSet.Add() returns false if the character
            // already exists in the set.
            if (!seen.Add(ch)) {
                return ch;
            }
        }

        // No repeated character found.
        return null;
    }
    private static void PrintResult(string input)
    {
        char? result = FindFirstRepeatedCharacter(input);
    
        Console.WriteLine(
            result.HasValue
                ? result.Value.ToString()
                : "No repeated character");
    }

    public static void Main()
    {
        PrintResult("abca");
        PrintResult("abcbd");
        PrintResult("abcde");
        PrintResult("abcdxypcbop");
    }
}
  
  
/*
run:
  
a
b
No repeated character
c
  
*/

 

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