Finding Longest Palindrome Given a File

Question

Hello Everyone,
I am tackling an algorithmic problem trying to find the longest palindrome inside of a text file. As of right now, I know how to determine if a word is or is not a palindrome. However, the issue that I am having at the moment is trying to come up with an efficient algorithm that can solve this in the fastest way possible. Please help give ideas/suggestions on an algorithm that I should follow to achieve this.
So for example, lets say we have a text file that has 10,000 characters. One of the things that I have concluded is that in order to determine if a word is a palindrome, you must be able to know the first and last characters. Right now, the only thing that I can think of is to:
for( start with the first character x,  until the last character 10,0000)
          int Y = index 10,000
          while( X < Y )
                if(the substring from X to the last character Y is not a palindrome)
                Y = Y - 1;
                else
                         currentLongestPalindrome = substring from X to Y;
                         BREAK;
}Sorry everyone for the incorrect Java syntax, but that is the basic idea that I have come up with. This looks terrible as it is taking over an hour to be able to run this solution. Do you guys have any suggestions?
Thanks in advance.

Answer 1

Hi,
I don't think you got the point of my main suggestion.
Just check each three letter sequence until you find what may be the centre of a palindrome then and only then expand your check
For your example
"DSAFJKLSAFDJDASFLJASDFLASJFKSJHCNXVABCDDCBAMXBCXBZNZQOEIQEWOUWQOUWQEPQWUE"
DSA
SAF
AFJ
etc
e.g.
DSA = FALSE
SAF = FALSE
FDJ = FALSE
DJD = TRUE -> F*DJD*A = FALSE - Current longest = 3
JDA = FALSE
DAS = FALSE
CDD = POSSIBLE -> CDDC = TRUE -> BCDDCB = TRUE -> ABCDDCBA = TRUE -> VABCDDCBAM = FALSE - Current longest = 8
DDC = POSSIBLE (no need to check less than 8) -> ABC*DDC*BA = TRUE -> VABCDDCBAM = FALSE - Current longest still 8
DCB = FALSE
etc
That sequence consists of 40 checks of three letter sequences and only needs to expand on 3 of them.
So on a 10,000 char file you would perform a max of 9,998 initial checks and then only expand on those that require it.
In an amendment to my initial suggestion sequence 'aaa' could also be the centre of a palindrome.
Hope this is clearer
Eric