merge: Fix IsPalindrome Big-O analysis (TheAlgorithms#1054)

* Fix IsPalindrome Big-O analysis

* Updated Documentation in README.md

Co-authored-by: github-actions <${GITHUB_ACTOR}@users.noreply.github.com>

Author: appgurueu

DIRECTORY.md

@@ -299,6 +299,7 @@
   * [FormatPhoneNumber](String/FormatPhoneNumber.js)
   * [GenerateGUID](String/GenerateGUID.js)
   * [HammingDistance](String/HammingDistance.js)
+  * [IsPalindrome](String/IsPalindrome.js)
   * [KMPPatternSearching](String/KMPPatternSearching.js)
   * [LevenshteinDistance](String/LevenshteinDistance.js)
   * [Lower](String/Lower.js)

String/IsPalindrome.js

@@ -10,7 +10,8 @@
   * Big-O Analysis
       * Time Complexity
         - O(N) on average and worst case scenario as input is traversed in linear fashion
-        - O(N) on best case scenario, even when input has length of 1, because toString() method takes O(N)
+        - O(1) on best case scenario if the input already is a string (otherwise toString() method takes O(N))
+               and the first & last characters don't match, triggering an early return
       * Space Complexity
         - O(1)
 */