Manacher algorithm for LPS

- SPOJ LPS problem worked fine until Test Case #15 got TLE
- Some optimization and cleanup pending

Author: c0D3M

manacher.cpp

@@ -0,0 +1,99 @@
+#include <bits/stdc++.h>
+#include <algorithm>
+using namespace std;
+
+long  long curren_p_len, current_palindrome_left=-1, current_palindrome_right=-1;
+ long  long max_p =0;
+void updateCurrentPalindrome( long  long index,   long  long length)
+{
+	 long  long offset = (length -1)/2;
+	curren_p_len = length;
+	current_palindrome_right = index+offset;
+	current_palindrome_left =  index-offset;
+	
+}
+ long  long findPalindromeLen( long  long centre,  long  long skip, string a)
+{
+	 long  long count =0;
+	while(centre-skip-count >=0 && (a[centre-skip-count]== a[centre+skip+count]))
+		count++;
+	if(count)
+		count --; //Skip the last offset
+	return (2*(skip+count)) +1;
+}
+ long  long LPS(string a)
+{
+	 long  long current_index = 0;
+	 vector<long  long> len;
+	 
+	while ( current_index < a.size())
+	{
+		cout << current_index<<endl;
+		if(max_p==a.size())
+			break;
+		if(current_index <= current_palindrome_right )
+		{
+			for ( long  long j = current_index -2 ; j>=current_palindrome_left; j--)
+			{
+				// Look for new centre i.e. Rule #3;left
+				 long  long offset = (len[j]-1)/2;
+				 long  long myleft = j - offset;
+				 long  long myright = current_index + offset;
+				if((myleft == current_palindrome_left) && (myright== current_palindrome_right))
+				{
+					//Found new centre, find palindrome
+					len.push_back(findPalindromeLen(current_index, offset, a));
+					updateCurrentPalindrome(current_index, len[current_index]);
+					break;
+				}
+				else if ((myleft>= current_palindrome_left) && (myright<= current_palindrome_right))
+				{
+				
+					//Current index totally lies in palindrom boundary ; just copy the palindrom length
+					len.push_back(len [j]);
+					current_index++;
+					//updateCurrentPalindrome(current_index, len[current_index]);
+				}
+				else
+				{
+					// Calculate length 
+					len.push_back( len[j] - (2* (current_palindrome_left-myleft)));
+					current_index++;
+				}
+			}
+		}
+		else
+		{
+			// Do normal
+			len.push_back(findPalindromeLen(current_index, 0, a));
+			updateCurrentPalindrome(current_index, len[current_index]);
+		}
+		if(max_p< len[current_index])
+			max_p =len[current_index];
+		current_index++;
+	}// while loop ends
+#ifdef DEBUG
+	for ( long  long i=0; i<a.size();i++)
+		cout << len[i]<<" ";
+#endif
+   return max_p;
+}
+int main(int argc, char*argv[])
+{
+	ios_base::sync_with_stdio(false);
+	cin.tie(NULL);
+	// long  long l;
+	//cin >>l;
+	string a;
+	cin >>a;
+	if(a.size()%2) // Odd len string
+		cout <<LPS(a);
+    else
+	{
+		string s;
+		for ( long  long i=0; i< (2*a.size())+1;i++)
+			s.push_back(i%2?a[i/2]:'$');
+		cout << LPS(s)/2;
+		
+	}
+}//$a$a$