Imports System
Imports System.Collections.Generic
Module Program
''' <summary>
''' Finds the first repeated character in a string.
'''
''' Examples:
''' "abca" -> "a"c
''' "abcbd" -> "b"c
''' "abcde" -> Nothing
''' "abcdxypcbop" -> "c"c
'''
''' Time Complexity: O(n)
''' Space Complexity: O(n)
''' </summary>
''' <param name="input">The input string.</param>
''' <returns>
''' The first repeated character, or Nothing if no repeated character exists.
''' </returns>
Function FindFirstRepeatedCharacter(input As String) As Char?
If String.IsNullOrEmpty(input) Then
Return Nothing
End If
Dim seen As New HashSet(Of Char)()
For Each ch As Char In input
' HashSet.Add returns False if the character
' already exists in the set.
If Not seen.Add(ch) Then
Return ch
End If
Next
' No repeated character found.
Return Nothing
End Function
Sub Main()
PrintResult("abca")
PrintResult("abcbd")
PrintResult("abcde")
PrintResult("abcdxypcbop")
End Sub
Private Sub PrintResult(input As String)
Dim result As Char? = FindFirstRepeatedCharacter(input)
If result.HasValue Then
Console.WriteLine(result.Value)
Else
Console.WriteLine("No repeated character")
End If
End Sub
End Module
' run:
'
' a
' b
' No repeated character
' c
'
