647. Palindromic Substrings
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.
Two PointersStringDynamic Programming
Hints
Hint 1
How can we reuse a previously computed palindrome to compute a larger palindrome?
Hint 2
If “aba” is a palindrome, is “xabax” a palindrome? Similarly is “xabay” a palindrome?
Hint 3
Complexity based hint:
If we use brute force and check whether for every start and end position a substring is a palindrome we have O(n^2) start - end pairs and O(n) palindromic checks. Can we reduce the time for palindromic checks to O(1) by reusing some previous computation?
If we use brute force and check whether for every start and end position a substring is a palindrome we have O(n^2) start - end pairs and O(n) palindromic checks. Can we reduce the time for palindromic checks to O(1) by reusing some previous computation?
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