updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -7,6 +7,7 @@
   * [N Queens All Solution Optimised](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/backtracking/n_queens_all_solution_optimised.cpp)
   * [Nqueen Print All Solutions](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/backtracking/nqueen_print_all_solutions.cpp)
   * [Rat Maze](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/backtracking/rat_maze.cpp)
+  * [Subarray Sum](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/backtracking/subarray_sum.cpp)
   * [Sudoku Solve](https://github.com/TheAlgorithms/C-Plus-Plus/blob/master/backtracking/sudoku_solve.cpp)
 
 ## Bit Manipulation