public static string FindLargestPalindrome(string s)
{
if (s.Length == 0)
{
return(string.Empty);
}
if (s.Length == 1)
{
return(s[0].ToString());
}
var rs = new string(s.Reverse().ToArray());
var ns = s + '\0' + rs;
var suffixTree = SuffixTreeNode.Build(ns);
var Earray = new int[(ns.Length + 2) * 4 - 1];
var Aarray = new int[(ns.Length + 2) * 4 - 1];
var Rarray = new int[ns.Length + 1];
suffixTree.WriteToArrays(Earray, 0, Aarray, Rarray);
var rmqHelper = new RMQPM1Helper(Aarray);
var lenMaxPalindrome = 0;
var midPalindrome = -1;
for (int i = 0; i < s.Length; i++)
{
var indL = Rarray[i + 1];
var indR = Rarray[ns.Length - i];
if (indL > indR)
{
int tmp = indL;
indL = indR;
indR = tmp;
}
int indexInE = rmqHelper.GetMinimum(indL, indR).Index;
var palindromeLength = Earray[indexInE];
if (palindromeLength > lenMaxPalindrome)
{
lenMaxPalindrome = palindromeLength;
midPalindrome = i + 1;
}
}
var startPalindrome = midPalindrome - lenMaxPalindrome;
var endPalindrome = startPalindrome + 2 * lenMaxPalindrome - 2;
var lenPalindrome = endPalindrome - startPalindrome + 1;
var r = s.Substring(startPalindrome, lenPalindrome);
return(r);
}
public static SuffixTreeNode Build(string s)
{
var s2 = '\t' + s + '\t';
var root = new SuffixTreeNode(0, 0, 0);
for (int i = 1; i < s2.Length - 1; i++)
{
var currentNode = root;
var indexInWord = i;
bool alreadyAdded = false;
while (currentNode.children.TryGetValue(s2[indexInWord], out SuffixTreeNode searchedChild))
{
int endMatchIndex = indexInWord;
int indInFound = searchedChild.startIndex;
while (s2[endMatchIndex] == s2[indInFound] && indInFound <= searchedChild.endIndex)
{
endMatchIndex++;
indInFound++;
}
bool wholeWord = indInFound == searchedChild.endIndex + 1;
if (!wholeWord)
{
var nodeOldChildren = new SuffixTreeNode(indInFound, searchedChild.endIndex, searchedChild.subwordStartIndex);
nodeOldChildren.children = searchedChild.children;
var newNode = new SuffixTreeNode(endMatchIndex, s2.Length - 1, i);
searchedChild.children = new Dictionary <char, SuffixTreeNode>
{
{ s2[indInFound], nodeOldChildren },
{ s2[endMatchIndex], newNode }
};
searchedChild.endIndex = indInFound - 1;
alreadyAdded = true;
break;
}
else
{
currentNode = searchedChild;
indexInWord = endMatchIndex;
}
}
if (!alreadyAdded)
{
var newNode = new SuffixTreeNode(indexInWord, s2.Length - 1, i);
currentNode.children.Add(s2[indexInWord], newNode);
}
}
root.UpdateDepth(0);
return(root);
}