docs: update DIRECTORY.md (#2810)

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DIRECTORY.md

@@ -302,6 +302,7 @@
   * [Kadanes3](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/kadanes3.cpp)
   * [Kelvin To Celsius](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/kelvin_to_celsius.cpp)
   * [Lfu Cache](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/lfu_cache.cpp)
+  * [Longest Substring Without Repeating Characters](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/longest_substring_without_repeating_characters.cpp)
   * [Lru Cache](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/lru_cache.cpp)
   * [Matrix Exponentiation](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/matrix_exponentiation.cpp)
   * [Palindrome Of Number](https://github.com/TheAlgorithms/C-Plus-Plus/blob/HEAD/others/palindrome_of_number.cpp)