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:
1 <= s.length <= 1000sconsists of lowercase English letters.
Solution
class Solution {
private func expand(_ a: [Character], _ l: Int, _ r: Int, _ res: inout Int) {
var l = l
var r = r
while l >= 0 && r < a.count {
if a[l] != a[r] {
return
} else {
res += 1
l -= 1
r += 1
}
}
}
func countSubstrings(_ s: String) -> Int {
let a = Array(s)
var res = 0
for i in 0..<a.count {
expand(a, i, i, &res)
expand(a, i, i + 1, &res)
}
return res
}
}