Move "valid parentheses" to the "Uncategorized" section

Author: trekhleb

README.md

@@ -144,8 +144,6 @@ a set of rules that precisely define a sequence of operations.
 * **Linked Lists**
   * `B` [Straight Traversal](src/algorithms/linked-list/traversal)
   * `B` [Reverse Traversal](src/algorithms/linked-list/reverse-traversal)
-* **Stack**
-  * `B` [Valid Parentheses](src/algorithms/stack/valid-parentheses)  - check if a string has valid parentheses in the correct order
 * **Trees**
   * `B` [Depth-First Search](src/algorithms/tree/depth-first-search) (DFS)
   * `B` [Breadth-First Search](src/algorithms/tree/breadth-first-search) (BFS)
@@ -188,6 +186,7 @@ a set of rules that precisely define a sequence of operations.
   * `B` [Rain Terraces](src/algorithms/uncategorized/rain-terraces) - trapping rain water problem (dynamic programming and brute force versions)
   * `B` [Recursive Staircase](src/algorithms/uncategorized/recursive-staircase) - count the number of ways to reach to the top (4 solutions)
   * `B` [Best Time To Buy Sell Stocks](src/algorithms/uncategorized/best-time-to-buy-sell-stocks) - divide and conquer and one-pass examples
+  * `B` [Valid Parentheses](src/algorithms/stack/valid-parentheses) - check if a string has valid parentheses (using stack)
   * `A` [N-Queens Problem](src/algorithms/uncategorized/n-queens)
   * `A` [Knight's Tour](src/algorithms/uncategorized/knight-tour)