[Java] Easy to understand - Postorder traversal
https://github.com/Freeze777/SDE-Interviewer-Notes/blob/main/LeetCodeJava/src/main/java/leetcode/medium/trees/DeleteNodesForest.java
https://github.com/Freeze777/SDE-Interviewer-Notes/blob/main/LeetCodeJava/src/main/java/leetcode/medium/trees/DeleteNodesForest.java
['Binary Tree', 'Java']