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Tree traversal refers to the process of visiting all the nodes of a tree in a specific order. There are three main techniques for tree traversal:

1. Preorder Traversal: Visiting the current node first, then traversing the left subtree, and finally the right subtree.

2. ___: Traversing the left subtree first, then visiting the current node, and finally the right subtree.

3. Postorder Traversal: Traversing the left subtree first, then the right subtree, and finally visiting the current node.

These traversal techniques can be used to perform various operations on tree nodes, such as printing the node values, searching for a specific value, or evaluating an expression tree.