[LeetCode] Longest Palindromic Substring

// O(n^2) time and O(1) space
// Given a center, either one letter or two letter,
// Find longest palindrome
private String expandCenter(String s, int left, int right) {
while (left >= 0 && right < s.length() && s.charAt(left) == s.charAt(right)) {
--left;
++right;
}
++left;
return s.substring(left, right);
}

public int longestPalindromicSubstring(String s) {
if (s == null || s.isEmpty()) return 0; //return "" here if it wants the actual string
String max = s.substring(0,1);
for (int i = 0; i < s.length()-1; ++i) { //it could be no "-1", also correct
// get longest palindrome with center of i, i+1
String palindrome = expandCenter(s, i, i+1);
max = palindrome.length() > max.length() ? palindrome : max;
// get longest palindrome with center of i
palindrome = expandCenter(s, i, i);
max = palindrome.length() > max.length() ? palindrome : max;
}
return max.length();//return max here if it wants  the max string
}

int p[1000], mx = 0, id = 0;
memset(p, 0, sizeof(p));
for (i = 1; s[i] != '\0'; i++) {
p[i] = mx > i ? min(p[2*id-i], mx-i) : 1;
while (s[i + p[i]] == s[i - p[i]]) p[i]++;
if (i + p[i] > mx) {
mx = i + p[i];
id = i;
}
}