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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
# @param {String} s
# @return {Integer}
def count_substrings(s)
a = s.chars
res = [0]
(0...a.length).each do |i|
expand_count(a, i, i, res)
expand_count(a, i, i + 1, res)
end
res[0]
end
def expand_count(a, l, r, res)
while l >= 0 && r < a.length
if a[l] != a[r]
return
else
res[0] += 1
l -= 1
r += 1
end
end
end