g0601_0700.s0647_palindromic_substrings.Solution Maven / Gradle / Ivy

Go to download

Show more of this group Show more artifacts with this name
Show all versions of leetcode-in-java21 Show documentation

Java-based LeetCode algorithm problem solutions, regularly updated

There is a newer version: 1.38

package g0601_0700.s0647_palindromic_substrings;

// #Medium #Top_100_Liked_Questions #String #Dynamic_Programming #Big_O_Time_O(n^2)_Space_O(n)
// #2022_03_21_Time_2_ms_(98.77%)_Space_41.7_MB_(75.10%)

/**
 * 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:**
 *
 * *   `1 <= s.length <= 1000`
 * *   `s` consists of lowercase English letters.
**/
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];
    }
}