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String•Pattern: Expand Around Center•Medium

Longest palindromic substring

Problem Intuition & Objective

Find the longest contiguous palindromic substring in a string.

Time ComplexityO(n²)
Optimal
Space ComplexityO(1)
Memory Bounds
Optimal Algorithmic Approach

Expand-around-center: 2n-1 possible centers, O(n) each in worst case.

Core Invariants & Edge Cases

  • Expand-around-center: for each index expand both odd and even centers
  • Track best start and length seen
  • DP is O(n²) time & space; expansion is O(n²) time O(1) space
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