Palindromic Substrings

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Problem

Given a string s, return the number of substrings of s (including single characters) that are palindromes, counting different positions as different substrings even when the text is identical.

Examples

Example 1

Input:s = "abc"

Output:3

"a", "b", "c".

Example 2

Input:s = "aaa"

Output:6

"a","a","a","aa","aa","aaa".

Constraints

1 <= s.length <= 1000

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How to approach it: the 1-D Dynamic Programming pattern

Break a problem into overlapping subproblems along a single dimension — like "the answer up to index i" — solve each one once, and reuse the result instead of recomputing it.

Look for this pattern when

A greedy or purely recursive approach recomputes the same subproblem repeatedly.
The problem asks for a count, a minimum/maximum, or a yes/no reachability over a sequence, where the answer at position `i` depends on answers at earlier positions.

Read the full 1-D Dynamic Programming guide →

Video walkthroughs

Original problem on LeetCode ↗