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.

解題

也可以用馬拉車算法
func countSubstrings(s string) int {
ans := 0
​
for i := 0; i < len(s); i++ {
ans += expand(s, i, i)
ans += expand(s, i, i + 1)
}
​
return ans
}
​
func expand(s string, start int, end int) int {
count := 0
​
for start >= 0 && end <= len(s) - 1 {
if s[start] != s[end] {
break
}
count++
start--
end++
}
​
return count
}