Reverse the string and perform longest common substring algorithm

Struggling through some of these problems so I can prepare myself. I kind of worked something out. It seems to do it in O(n), but the code turned out so simple that I am skeptical I actually got it. Syntax of some things may not even be right. Anyways, here's my solution. public static String longestP (String mystring) { HashMap myhash = new HashMap(); int temp1, temp2; int i; int palindromeLength = 0; String longestPalindrome; for (i=0;i palindromeLength)) { palindromeLength = substring.length(); longestPalindrome = substring; } } } return longestPalindrome; }