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CS 261 – Recitation 9 & 10 Graphs & Final review Fall 2013 Oregon State University School of Electrical Engineering and Computer Science

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Graph: Reachability Problem Given a single starting vertex, produce the set of vertices that can be reached starting from the initial location. A depth-first search follows each path as far (deep) as possible before backtracking. A breadth-first search looks at all possible paths at the same time. Order in which nodes are reached: (left) DFS; and (right) BFS. Source: Wikipedia

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Graph: Reachability Problem findReachable (graph g, vertex start) { create a set of reachable vertices, initially empty. call this r. create a container for vertices known to be reachable. call this c add start vertex to container c while the container c is not empty { remove first entry from the container c, assign to v if v is not already in the set of reachable vertices r { add v to the reachable set r add the neighbors of v to the container c } return r } DFS: Container is a Stack BFS: Container is a Queue

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Exercise Simulate the DFS and BFS on the following graph starting at node A. Notes: (1) Nodes must be added to the container (Stack or Queue) in COUNTER-CLOCKWISE order; (2) We do not add neighbors to the container if they have already been visited.

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CS 261 – Data Structures Outline – Final exam review BST/AVL/Tree sort/Tree traversal/Tree iterator Heaps/heap sort Hash tables Materials in these slides were collected from different Internet sources. 5

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CS 261 – Data Structures Question 6 H J D C G B K F A I E

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Pre, In, Post-order traversal Pre-order: 10 – 5 – 1 – 8 – 7 – 6 – 34 – 56 – 40 - 60 In-order: 1 – 5 – 6 – 7 – 8 – 10 – 34 – 40 – 56 - 60 Post–order: 1 – 6 – 7 – 8 – 5 – 40 – 60 – 56 – 34 – 10 CS 261 – Data Structures7

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Adding 13??? CS 261 – Data Structures8 13

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Removing 10??? CS 261 – Data Structures9

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TreeSort struct AVLTree* newAVLTree(); void addAVLTree(struct AVLTree *tree, TYPE val); void treeSort (TYPE data[], int n) {…} void _treeSortHelper(AVLNode *cur, TYPE *data, int *count) {…} CS 261 – Data Structures10

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treeSort void treeSort(TYPE data[], int n){ int i; int count = 0; /* declare an AVL tree */ struct AVLTree *tree = newAVLtree(); assert(data != NULL && n > 0); /* add elements to the tree */ for (i = 0; i < n; i++) addAVLTree(tree, data[i]); /* call the helper function */ _treeSortHelper(tree->root, data, &count); } CS 261 – Data Structures11

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_treeSortHelper /* *count goes from 0 to n-1 */ void _treeSortHelper(AVLNode *cur, TYPE *data, int *count){ if (cur != NULL) { _treeSortHelper(cur->left, data, count); data[*count] = cur->val; (*count)++; _treeSortHelper(cur->right, data, count); } } CS 261 – Data Structures12

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True or False CS 261 – Data Structures13

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Question Add 12, remove 3, remove 5 CS 261 – Data Structures14

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When to do a double rotation? Balance Factor = height(right subtree) - height(left subtree) At an unbalanced node N, a double rotation is needed when: –N’s BF is positive and N’s right subtree’s BF is negative –N’s BF is negative and N’s left subtree’s BF is positive. CS 261 – Data Structures15

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Draw the stack of in-order traversal using iterator if stack is empty perform slide left on root otherwise let n be top of stack pop n slide left on right child of n CS 261 – Data Structures16

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Heaps and priority queues How to represent a binary heap? –Using an array (dyArray) Suppose the root has index 0, what are the indices of the 2 children of a node at index i? What is the index of the parent of a node at index i? CS 261 – Data Structures 17 2 5 8 3 7910 14121116 0202 1313 2525 3939 4 10 5757 6868 7 14 8 12 9 11 10 16 2 * i + 1, 2 * i + 2 (i-1)/2

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Simulate heap sort for the following heap CS 261 – Data Structures18 3 9 10 141211 16

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