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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 <= 1000sconsists of lowercase English letters.
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 }