Palindromic Substrings

LeetCode Q 647 - Palindromic Substrings

Given a string, your task is to count how many palindromic substrings in this string. The substrings with different start indexes or end indexes are counted as different substrings even they consist of same characters.

Example 1:
Input: "abc" ; Output: 3
Explanation: Three palindromic strings: “a”, “b”, “c”.
Example 2:
Input: "aaa" ; Output: 6
Explanation: Six palindromic strings: “a”, “a”, “a”, “aa”, “aa”, “aaa”.

Note: The input string length won’t exceed 1000.

Solution : Expand Around Center

Similar Question: Longest Palindromic Substring

Code:

private int num; 
public int countSubstrings(String s) {
	if (s == null || s.length() == 0) return 0;
	for (int i = 0; i + 1< s.length(); i++) {
		expandAroundCerter(s, i, i);
		expandAroundCerter(s, i, i + 1);
	}
	
	return num + 1;
}
	
private void expandAroundCerter(String s, int start, int end) {
	while (start >= 0 && end < s.length() 
			&& s.charAt(start) == s.charAt(end)) {
	  	start--; end++; num++;
	}
}