Postorder Traversal

Overview

Postorder Traversal is a Tree DFS where the order is: Left Child, Right Child, Current Node. The parent is processed absolutely last. This is exactly what you need when you are deleting a tree, because you cannot delete a parent node until all of its children have safely been deleted first.

Syntax

Leaves are processed first. The root of the entire tree is printed absolutely last.

Left, Right, Root

java

public void postorder(TreeNode root) {
    if (root == null) return;
    
    postorder(root.left);       // 1. Left
    postorder(root.right);      // 2. Right
    System.out.print(root.val); // 3. Root (Process)
}

Common Pitfalls

It is the hardest of the three traversals to implement iteratively. Stick to recursion if possible!

Interview Tips

Postorder means the 'Root' is processed POST (after) its children.

Real-World Example

Evaluating Mathematical Syntax Trees (like 3 * (4 + 5)). You must evaluate the child operations (4 + 5) before applying the parent operation (* 3).

example

java

// Postorder evaluation of syntax trees ensures proper order of operations.

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