
May 23rd, 2012, 07:49 AM
#1
Finding the 3 longest palindromes
Hello guys,
as a novice C# programmer (I come from MatLab and Java) I'm being asked to create an algorithm that finds, given a string, the three longest palindromes contained in it, and returns them, with their starting index and length. For example, the output for string, “sqrrqabccbatudefggfedvwhijkllkjihxymnnmzpop” should be:
Text: hijkllkjih, Index: 23, Length: 10
Text: defggfed, Index: 13, Length: 8
Text: abccba, Index: 5 Length: 6
I'm downloading #develop now, and I'll give it a go, but does anybody have any ideas/solutions? Thanks

May 23rd, 2012, 08:02 AM
#2
Re: Finding the 3 longest palindromes
Just writing my draft ideas on topic :
Finding start of palindrom just going from left to right can be problematic in my mind as you would have to check **** tons of combinaisons.
Though you can reduce by reading character after character your string and check for occurences of 2 times the same letter. Why ?
Minimal palindrome size is "aa".
So here's my idea in pseudo code
Code:
Function 1 (string input)
{
for each character in input compare to the next
if the 2 characters are equal
then execute function 2
loop
return results
}
Function2 (int position_start)
{
this function will check both left and right character for a given position (+1 for a give, depends if the pos start is the left or right character of initial couple) and move one character far from the start each loop
loop while they are equal
return length and index (as you'll have it)
}
After that c# is quite simillar to Java so you shouldn't have too many problems.

May 23rd, 2012, 08:49 AM
#3
Re: Finding the 3 longest palindromes
Thanks Erendar. Your first thoughts matched mine. Though I did some more reading, and it turns out there is much more. Our basic assumption, that the minimal palindrome is "aa", is wrong, because that is only true if we consider a n empty space to be the only possible center. But actually, a letter may also be a center, so "a" is the minimal palindrome.
After that, there is tons more to consider. It is easy to solve in O(3), more difficult in O(2). People have found a linear time solution (fantastic!), documented at http://www.akalin.cx/longestpalindromelineartime for your own curiosity.
Thank you very much
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