MEDIUM NC#104 Blind #58 Dynamic Programming / Expand Around Center
647. Palindromic Substrings
📖 Problem
Given a string s, return the number of palindromic substrings in s. A palindrome string is a string that reads the same backward as forward.
🧠 Visual Learning Aid
1 Model the input into the right structure
2 Choose the core technique and invariant
3 Execute step-by-step with a sample
4 Validate complexity and edge cases
JS/TS Refreshers
- •Array methods (`push`, `pop`, `shift`, `slice`)
- •Object/Map/Set usage patterns
- •Function parameter and return typing
- •Array DP table updates
- •State transition thinking
- •Base case initialization
Logical Thinking Concepts
- •Define invariants before coding
- •Check edge cases first (`[]`, single element, duplicates)
- •Estimate time/space before implementation
- •Apply Dynamic Programming reasoning pattern
- •Apply Recursion reasoning pattern
💡 Approach
- → Use expand around center technique: each character and gap can be a center
- → Expand from each center and count palindromes
- → Alternatively, use DP to build palindrome table
- → Time: O(n²), Space: O(1) for expand, O(n²) for DP
🛠️ Hints & Pitfalls
Hints
- •Use expand around center technique: each character and gap can be a center
- •Expand from each center and count palindromes
- •Alternatively, use DP to build palindrome table
Common Pitfalls
- •Time: O(n²), Space: O(1) for expand, O(n²) for DP
🧪 Test Cases
Hidden tests on submit: 3
Test Case 1
Not run Input:
countSubstrings('abc'); Expected:
3 Test Case 2
Not run Input:
countSubstrings('aaa'); Expected:
6 Test Case 3
Not run Input:
countSubstrings('ababa'); Expected:
9 📝 Code Editor
📤 Output