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the current node's left subtree.
# Recursively traverse the current node's right subtree.

The pre-order traversal is a topologically sorted one, because a parent node is processed before any of its child nodes is done.

Post-order, LRN

# Recursively traverse the current node's left subtree.
# Recursively traverse the current node's right subtree.
# Visit the current node (in the figure: position blue).

Post-order traversal can be useful to get postfix expression

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