private static void UpdateLongestPalindromes(LongestUniquePalindromes longestPalindromes, HashSet <string> nonUniquePalindromes, PalindromeData candidatePalindrome)
{
//if the candidate palindrome is strictly contained within another palindrome, it is not longest, or not unique: continue
if (longestPalindromes.IntervalContains(candidatePalindrome))
{
return;
}
//if the candidate palindrome has already been recognized as not unique, continue.
if (nonUniquePalindromes.Contains(candidatePalindrome.Palindrome))
{
return;
}
if (!longestPalindromes.Contains(candidatePalindrome))
{
longestPalindromes.Add(candidatePalindrome);
}
else
{
//a palindrome of an equal length but different interval has been encountered before, it is not unique.
//IMPORTANT: it also means that the palindrome that we have encountered is not unique as well.
//remove it from the longest palindromes, and recognize it as non-unique.
longestPalindromes.Remove(candidatePalindrome);
nonUniquePalindromes.Add(candidatePalindrome.Palindrome);
}
}
/// <summary>
/// The main function to compute unique longest palindromes
/// </summary>
/// <param name="numOfPalindromes"> a positive integer </param>
/// <param name="s"> the string to search palindromes in. Not null</param>
/// <returns>a data structrue LongestUniquePalindromes containing at most numOfPalindromes unique longest palindromes </returns>
public static LongestUniquePalindromes GetLongestUniquePalindromes(int numOfPalindromes, string s)
{
if (s == null)
{
throw new ArgumentNullException("s");
}
if (numOfPalindromes < 1)
{
throw new ArgumentOutOfRangeException("numOfPalindromes");
}
LongestUniquePalindromes longestPalindromes = new LongestUniquePalindromes();
HashSet <string> nonUniquePalindromes = new HashSet <string>();
bool foundAllPalindromes = false;
//in decreasing string length
for (int i = s.Length; i > 0; i--)
{
//there are (s.Length-i+1) possible partitions of length i. examine whether they are palindromes.
int numberOfPartitions = s.Length - i + 1;
for (int j = 0; j < numberOfPartitions; j++)
{
string substringToCheck = s.Substring(j, i);
bool isPalindrome = PalindromeRecognizer.IsPalindrome(substringToCheck);
if (isPalindrome)
{
PalindromeData candidatePalindrome = new PalindromeData(j, i, substringToCheck);
UpdateLongestPalindromes(longestPalindromes, nonUniquePalindromes, candidatePalindrome);
}
}
//we have examined here all palindromes of length i.
//check now if we have sufficient number of palindromes to return
//IMPORTANT: we must traverse over all partitions of length i, in order to make
//sure that we capture only the unique palindromes of that length.
if (longestPalindromes.Count >= numOfPalindromes)
{
foundAllPalindromes = true;
longestPalindromes.RemoveRange(numOfPalindromes, longestPalindromes.Count - numOfPalindromes);
break;
}
}
//if we have not reached numOfPalindromes, add the empty string (a palindrome of length 0)
//and return with whatever number of longest palindrome we have found.
//i.e., Min ( longestPalindrome.Count, numOfPalindromes).
if (!foundAllPalindromes)
{
PalindromeData candidatePalindrome = new PalindromeData(0, 0, string.Empty);
longestPalindromes.Add(candidatePalindrome);
}
return(longestPalindromes);
}