// Return the first cell in the second half of the list.
// If the list has an odd number of cells and
// countMiddleCell is true, return the middle cell.
// If the list has an odd number of cells and
// countMiddleCell is false, return the cell after the middle.
public LetterCell FindSecondHalf(bool countMiddleCell)
{
// Find the list's halfway point.
LetterCell slowCell = this;
LetterCell fastCell = slowCell;
for (; ;)
{
// Move slowCell.
slowCell = slowCell.Next;
// Move fastCell.
fastCell = fastCell.Next;
if (fastCell == null)
{
// The list has an even number of items and
// slowCell is the first cell in the second half.
return(slowCell);
}
fastCell = fastCell.Next;
if (fastCell == null)
{
// The list has an odd number of items
// and slowCell is the middle item.
if (!countMiddleCell)
{
slowCell = slowCell.Next;
}
return(slowCell);
}
}
}
// Use a reversed list to return true if the
// cells after this one form a palindrome.
public bool IsPalindromeReverse()
{
// Make a reversed list.
LetterCell newTop = new LetterCell();
newTop.Next = null;
for (LetterCell oldCell = this.Next; oldCell != null; oldCell = oldCell.Next)
{
LetterCell newCell = new LetterCell();
newCell.Letter = oldCell.Letter;
newCell.Next = newTop.Next;
newTop.Next = newCell;
}
// Compare the lists.
LetterCell cell1 = this.Next;
LetterCell cell2 = newTop.Next;
while (cell1 != null)
{
if (cell1.Letter != cell2.Letter)
{
return(false);
}
cell1 = cell1.Next;
cell2 = cell2.Next;
}
return(true);
}
// Return a string representing this
// cell and those that follow.
public override string ToString()
{
string result = "";
for (LetterCell cell = this; cell != null; cell = cell.Next)
{
result += cell.Letter;
}
return(result);
}
// Recursively see if the
// cells after this one form a palindrome.
public bool IsPalindromeRecursive()
{
// Find the start of the second half.
LetterCell secondHalf = FindSecondHalf(false);
// Recursively compare the second and first halves.
LetterCell firstHalf = this.Next;
return(CompareHalves(ref firstHalf, secondHalf));
}
// See if the string is a palindrome.
private void checkButton_Click(object sender, EventArgs e)
{
// Make the string into a list.
string txt = stringTextBox.Text.ToUpper();
txt = RemoveNonLetters(txt);
// Make it into a list.
LetterCell firstCell = null;
foreach (char ch in txt)
{
LetterCell cell = new LetterCell();
cell.Letter = ch;
cell.Next = firstCell;
firstCell = cell;
}
// Add a sentinel.
LetterCell sentinel = new LetterCell();
sentinel.Next = firstCell;
// Check for a palindrome in various ways.
if (sentinel.IsPalindromeReverse())
{
reverseLabel.Text = "Is a palindrome";
}
else
{
reverseLabel.Text = "Not a palindrome";
}
if (sentinel.IsPalindromeReverseHalf())
{
reverseHalfLabel.Text = "Is a palindrome";
}
else
{
reverseHalfLabel.Text = "Not a palindrome";
}
if (sentinel.IsPalindromeRecursive())
{
recursiveLabel.Text = "Is a palindrome";
}
else
{
recursiveLabel.Text = "Not a palindrome";
}
}
// Make a list to represent a string.
public LetterCell(string txt)
{
// Make it into a list.
LetterCell firstCell = null;
foreach (char ch in txt)
{
LetterCell cell = new LetterCell();
cell.Letter = ch;
cell.Next = firstCell;
firstCell = cell;
}
// Make this cell be the sentinel.
Next = firstCell;
}
// Recursively compare the second and first halves.
private bool CompareHalves(ref LetterCell firstHalf, LetterCell secondHalf)
{
// See if we're at the end of the second half.
if (secondHalf.Next == null)
{
// Compare the last cell in the second half
// to the first cell in the first half.
bool ourResult = (secondHalf.Letter == firstHalf.Letter);
// Move firstHalf to the next item.
firstHalf = firstHalf.Next;
// Return so the next level up can continue checking.
return(ourResult);
}
// We are not at the end of the second half.
// Recursively check one cell farther the second half.
// If the recursive call returns false,
// this is not a palindrome.
if (!CompareHalves(ref firstHalf, secondHalf.Next))
{
return(false);
}
// Compare the next cells in the two halves.
if (firstHalf.Letter != secondHalf.Letter)
{
return(false);
}
// If we get this far, it is a palindrome so far.
// Move firstHalf to the next item.
firstHalf = firstHalf.Next;
// So far so good.
return(true);
}
// Use half a reversed list to return true if the
// cells after this one form a palindrome.
public bool IsPalindromeReverseHalf()
{
// Lists with 0 or 1 letters are palindromes.
if ((this.Next == null) || (this.Next.Next == null))
{
return(true);
}
// Find the list's halfway point.
LetterCell secondHalf = FindSecondHalf(false);
// Make a reversed list.
LetterCell lastCell = null;
for (LetterCell oldCell = secondHalf; oldCell != null; oldCell = oldCell.Next)
{
LetterCell newCell = new LetterCell();
newCell.Letter = oldCell.Letter;
newCell.Next = lastCell;
lastCell = newCell;
}
// Compare the lists.
LetterCell cell1 = lastCell;
LetterCell cell2 = this.Next;
while (cell1 != null)
{
if (cell1.Letter != cell2.Letter)
{
return(false);
}
cell1 = cell1.Next;
cell2 = cell2.Next;
}
return(true);
}
// Parameterless constructor.
public LetterCell()
{
Next = null;
}
