updating DIRECTORY.md

Author: github-actions

DIRECTORY.md

@@ -75,6 +75,7 @@
  * Stacks
  * [BalancedBrackets](https://github.com/TheAlgorithms/Java/blob/master/DataStructures/Stacks/BalancedBrackets.java)
  * [DecimalToAnyUsingStack](https://github.com/TheAlgorithms/Java/blob/master/DataStructures/Stacks/DecimalToAnyUsingStack.java)
+ * [InfixToPostfix](https://github.com/TheAlgorithms/Java/blob/master/DataStructures/Stacks/InfixToPostfix.java)
  * [NodeStack](https://github.com/TheAlgorithms/Java/blob/master/DataStructures/Stacks/NodeStack.java)
  * [StackArray](https://github.com/TheAlgorithms/Java/blob/master/DataStructures/Stacks/StackArray.java)
  * [StackArrayList](https://github.com/TheAlgorithms/Java/blob/master/DataStructures/Stacks/StackArrayList.java)