LeetCode in Kotlin

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.

Solution

class Solution {
    private fun expand(a: CharArray, l: Int, r: Int, res: IntArray) {
        var left = l
        var right = r
        while (left >= 0 && right < a.size) {
            if (a[left] != a[right]) {
                return
            } else {
                res[0]++
                left--
                right++
            }
        }
    }

    fun countSubstrings(s: String): Int {
        val a = s.toCharArray()
        val res = intArrayOf(0)
        for (i in a.indices) {
            expand(a, i, i, res)
            expand(a, i, i + 1, res)
        }
        return res[0]
    }
}

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