Lec21
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The document discusses different methods for traversing binary trees, including preorder, inorder, postorder, and level order traversal. It explains that during traversal, each node is visited exactly once and some operation is performed on that node. It provides examples of traversing different binary trees using each method and discusses how inorder and preorder traversal sequences can uniquely define the structure of a binary tree.
Lec21
- 1. Binary Tree Traversal Methods • In a traversal of a binary tree, each element of the binary tree is visited exactly once. • During the visit of an element, all action (make a clone, display, evaluate the operator, etc.) with respect to this element is taken.
- 2. Binary Tree Traversal Methods • Preorder • Inorder • Postorder • Level order
- 3. Preorder Traversal public static void preOrder(BinaryTreeNode t) { if (t != null) { visit(t); preOrder(t.leftChild); preOrder(t.rightChild); } }
- 4. Preorder Example (visit = print) a b c a b c
- 5. Preorder Example (visit = print) a b c d e f g h i j a b d g h e i c f j
- 6. Preorder Of Expression Tree + a b - c d + e f * / / * + a b - c d + e f Gives prefix form of expression!
- 7. Inorder Traversal public static void inOrder(BinaryTreeNode t) { if (t != null) { inOrder(t.leftChild); visit(t); inOrder(t.rightChild); } }
- 8. Inorder Example (visit = print) a b c b a c
- 9. Inorder Example (visit = print) a b c d e f g h i j g d h b e i a f j c
- 10. Inorder By Projection (Squishing) a b c d e f g h i j g d h b e i a f j c
- 11. Inorder Of Expression Tree + a b - c d + e f * / a + b * c - d / e + f Gives infix form of expression (sans parentheses)!
- 12. Postorder Traversal public static void postOrder(BinaryTreeNode t) { if (t != null) { postOrder(t.leftChild); postOrder(t.rightChild); visit(t); } }
- 13. Postorder Example (visit = print) a b c b c a
- 14. Postorder Example (visit = print) a b c d e f g h i j g h d i e b j f c a
- 15. Postorder Of Expression Tree + a b - c d + e f * / a b + c d - * e f + / Gives postfix form of expression!
- 16. Traversal Applications a b c d e f g h i j • Make a clone. • Determine height. •Determine number of nodes.
- 17. Level Order Let t be the tree root. while (t != null) { visit t and put its children on a FIFO queue; remove a node from the FIFO queue and call it t; // remove returns null when queue is empty }
- 18. Level-Order Example (visit = print) a b c d e f g h i j a b c d e f g h i j
- 19. Binary Tree Construction • Suppose that the elements in a binary tree are distinct. • Can you construct the binary tree from which a given traversal sequence came? • When a traversal sequence has more than one element, the binary tree is not uniquely defined. • Therefore, the tree from which the sequence was obtained cannot be reconstructed uniquely.
- 20. Some Examples preorder = ab a b a b inorder = ab b a a b postorder = ab b a b a level order = ab a b a b
- 21. Binary Tree Construction • Can you construct the binary tree, given two traversal sequences? • Depends on which two sequences are given.
- 22. Preorder And Postorder preorder = ab a b a postorder = ba b • Preorder and postorder do not uniquely define a binary tree. • Nor do preorder and level order (same example). • Nor do postorder and level order (same example).
- 23. Inorder And Preorder • inorder = g d h b e i a f j c • preorder = a b d g h e i c f j • Scan the preorder left to right using the inorder to separate left and right subtrees. • a is the root of the tree; gdhbei are in the left subtree; fjc are in the right subtree. a gdhbei fjc
- 24. Inorder And Preorder a gdhbei fjc • preorder = a b d g h e i c f j • b is the next root; gdh are in the left subtree; ei are in the right subtree. a b fjc ei gdh
- 25. Inorder And Preorder a b fjc ei gdh • preorder = a b d g h e i c f j • d is the next root; g is in the left subtree; h is in the right subtree. a g b fjc d ei h
- 26. Inorder And Postorder • Scan postorder from right to left using inorder to separate left and right subtrees. • inorder = g d h b e i a f j c • postorder = g h d i e b j f c a • Tree root is a; gdhbei are in left subtree; fjc are in right subtree.
- 27. Inorder And Level Order • Scan level order from left to right using inorder to separate left and right subtrees. • inorder = g d h b e i a f j c • level order = a b c d e f g h i j • Tree root is a; gdhbei are in left subtree; fjc are in right subtree.
Editor's Notes
- #4: t is the root of the subtree being traversed.
- #6: During the traversal, t starts at the root, moves to the left child b of the root, then to the left child d of b. When the traversal of the left subtree of b is complete, t, once again, points to the node b. The t moves into the right subtree of b. When the traversal of this right subtree is complete, t again points to b. Following this, t points to a. We see that t points to every node in the binary tree three times – once when you get to the node from its parent (or in the case of the root, t is initially at the root), once when you return from the left subtree of the node, and once when you return from the node’s right subtree. Of these three times that t points to a node, the node is visited the first time.
- #17: Make a clone using postorder traversal … clone the left subtree, clone the right subtree, clone the root in the visit step. Determine height using postorder traversal … determine the height of the left subtree, determine the height of the right subtree, in the visit step add 1 to the max of the already determined heights of the left and right subtrees. Determine number of nodes using preorder, inorder, or postorder traversal … initialize a counter to 0, add 1 to the counter in the visit step.