LeetCode-in-All

647. Palindromic Substrings

Medium

Given a string s, return the number of palindromic substrings in it.

A string is a palindrome when it reads the same backward as forward.

A substring is a contiguous sequence of characters within the string.

Example 1:

Input: s = “abc”

Output: 3

Explanation: Three palindromic strings: “a”, “b”, “c”.

Example 2:

Input: s = “aaa”

Output: 6

Explanation: Six palindromic strings: “a”, “a”, “a”, “aa”, “aa”, “aaa”.

Constraints:

Solution

public class Solution {
    private void Expand(char[] a, int l, int r, int[] res) {
        while (l >= 0 && r < a.Length) {
            if (a[l] != a[r]) {
                return;
            } else {
                res[0]++;
                l--;
                r++;
            }
        }
    }

    public int CountSubstrings(string s) {
        char[] a = s.ToCharArray();
        int[] res = {0};
        for (int i = 0; i < a.Length; i++) {
            Expand(a, i, i, res);
            Expand(a, i, i + 1, res);
        }
        return res[0];
    }
}