I have the following code for calculating the longest palindrome sub string in the string. The online judge accepts O(n^2) solution but i don't know why it is not accepting my solution although it seems that my algorithm is O(n^2) in complexity.`
class Ideone {
public static void main(String args[]) {
Ideone ob = new Ideone();
String s = "sds";
System.out.println(ob.longestPalindrome(s));
}
public String longestPalindrome(String s) {
int maxlength = 1;
String ps = s.charAt(0) + "";
if (s.length() == 1)
return s;
for (int i = 0; i < s.length() - 1; i++) {
int j = (s.substring(i + 1)).indexOf(s.charAt(i)) + i + 1;
while (j < s.length() && j > i) {
if (j - i + 1 > maxlength && check(s.substring(i, j + 1))) {
maxlength = j - i + 1;
ps = s.substring(i, j + 1);
}
if ((s.substring(j + 1)).indexOf(s.charAt(i)) < 0) {
break;
}
j = (s.substring(j + 1)).indexOf(s.charAt(i)) + j + 1;
}
}
return ps;
}
public boolean check(String s) {
int l = s.length();
if (l == 1)
return false;
int t = l / 2;
String s1, s2;
if (l % 2 == 0) {
s1 = s.substring(0, t);
s2 = s.substring(t);
} else {
s1 = s.substring(0, t);
s2 = s.substring(t + 1);
}
s2 = (new StringBuffer(s2)).reverse().toString();
if (s1.compareTo(s2) == 0)
return true;
else return false;
}
}
At first glimpse two loops and a method check() which needs O(n) to reverse the String might lead to O(n³).
Be aware that methods like:
Need to iterate through the data and therefore need about O(n) and not constant time.