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

function expand(a: string[], l: number, r: number, res: number[]): void {
    while (l >= 0 && r < a.length) {
        if (a[l] !== a[r]) {
            return
        } else {
            res[0]++
            l--
            r++
        }
    }
}

function countSubstrings(s: string): number {
    const a: string[] = s.split('')
    const res: number[] = [0]
    for (let i = 0; i < a.length; i++) {
        expand(a, i, i, res)
        expand(a, i, i + 1, res)
    }
    return res[0]
}

export { countSubstrings }