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 function expand($a, $l, $r, &$res) {
while ($l >= 0 && $r < count($a)) {
if ($a[$l] != $a[$r]) {
return;
} else {
$res[0]++;
$l--;
$r++;
}
}
}
/**
* @param String $s
* @return Integer
*/
public function countSubstrings($s) {
$a = str_split($s);
$res = array(0);
for ($i = 0; $i < count($a); $i++) {
$this->expand($a, $i, $i, $res);
$this->expand($a, $i, $i + 1, $res);
}
return $res[0];
}
}