Add notes on palindrome algorithms

Added notes on palindrome algorithms including expand-around-center and Manacher's Algorithm.

leetcode/lc5/notes.md

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+The expand-around-center approach checks every character (and every gap between characters) as a potential palindrome center and expands outward to verify palindromes, resulting in repeated character comparisons and a worst-case time complexity of O(n²).
+
+In contrast, Manacher’s Algorithm optimizes this process by transforming the string and using palindrome symmetry to reuse previously computed information, thereby eliminating redundant expansions and achieving a linear time complexity of O(n).
+in the code 
+C -> center of the rightmost palindrome
+R -> right boundary of rightmost Palindrome