Data Structures Review Session 2 Ramakrishna, PhD student. Grading Assistant for this course CS 307 Fundamentals of Computer Science

Binary Search Trees A binary tree is a tree where each node has at most two children, referred to as the left and right child A binary search tree is a binary tree where every node's left subtree holds values less than the node's value, and every right subtree holds values greater. A new node is added as a leaf. root parent 17 11 19 right child left child CS 307 Fundamentals of Computer Science

Problems Problem 1: How do you sort n numbers using a binary Search tree ?? What are the best, worst and Average case time complexities ?? CS 307 Fundamentals of Computer Science

Binary Search Tree properties All dynamic-set operations (Search, Insert, Delete, Min, Max, successor, predecessor) can be supported in O(h) time. h = (lg n) for a balanced binary tree (and for an average tree built by adding nodes in random order.) h = (n) for an unbalanced tree that resembles a linear chain of n nodes in the worst case. Red-black trees are a variation of binary search trees to ensure that the tree is balanced. Height is O (log n), where n is the number of nodes. CS 307 Fundamentals of Computer Science

Balance of Binary Trees A binary tree can be balanced, such that one branch of the tree is about the same size and depth as the other. In order to find out if a tree node is balanced, you need to find out the maximum height level of both children in each node, and if they differ by more than one level, it is considered unbalanced. If the number is -1, 0, or 1, the node is balanced. If the difference is anything else, then it is unbalanced. Note that a balanced binary tree requires every node in the tree to have the balanced property. CS 307 Fundamentals of Computer Science

Sorting Using BST Inorder traversal of a binary search tree always gives a sorted sequence of the values. This is a direct consequence of the BST property. Given a set of unordered elements, the following method can be used to Sort the elements: construct a binary search tree whose keys are those elements, and then perform an inorder traversal of this tree. BSTSort(A) 1. for each element in an array do 2. Insert element in the BST// Constructing a BST take O( log n) time 3. Inorder-Traversal (root) // Takes O(n) time Best case running time of BSTSort(A) is O( n log n). Worst case running time is O(n2) since each inserting could take O(n) time in worst case. CS 307 Fundamentals of Computer Science

Example Sorting Using BST Input Sequence :- 2 3 8 4 5 7 6 1 2.5 2.4 Step 1 : Creating Binary Search Tree of above given input sequence. 2 3 1 8 2.5 4 2.4 5 7 CS 307 Fundamentals of Computer Science 6

Example Sorting Using BST (cont.) Input Sequence :- 2 3 8 4 5 7 6 1 2.5 2.4 Step 2 : Perform Inorder-Traversal. 2 1 2 3 1 8 2.5 4 2.4 5 7 6 CS 307 Fundamentals of Computer Science

Example Sorting Using BST (cont.) Input Sequence :- 2 3 8 4 5 7 6 1 2.5 2.4 Step 2 : Perform Inorder-Traversal. 2 1 2 3 1 2.4 2.5 3 8 2.5 4 2.4 5 7 6 CS 307 Fundamentals of Computer Science

Example Sorting Using BST (cont.) Input Sequence :- 2 3 8 4 5 7 6 1 2.5 2.4 Step 2 : Perform Inorder-Traversal. 2 1 2 3 1 2.4 2.5 3 8 2.5 4 5 6 4 2.4 7 8 5 Sorted Array 7 6 CS 307 Fundamentals of Computer Science

Worst cases Input Sequence :- 8 4 3 2 and 2 3 8 10 Step 1 : Creating Binary Search Tree of above given input sequence. 2 8 4 3 3 8 2 10 CS 307 Fundamentals of Computer Science

Finding Min & Max Tree-Minimum(x) Tree-Maximum(x) The binary-search-tree property guarantees that: The minimum is located at the left-most node. The maximum is located at the right-most node. Tree-Minimum(x) Tree-Maximum(x) 1. while left[x]  NIL 1. while right[x]  NIL 2. do x  left[x] 2. do x  right[x] 3. return x 3. return x Q: How long do they take? CS 307 Fundamentals of Computer Science

Algorithm LCA (Node v, Node w): int vdpth  v.depth int wdpth  w.depth while vdpth > wdpth do v  v.parent vdpth  vdpth -1 end while while wdpth > vdpth do w  w.parent wdpth  wdpth -1 while v ≠ w do return v Note that LCA algorithm is applicable for any tree CS 307 Fundamentals of Computer Science

Give an efficient algorithm for converting an infix arithmetic expression into its equivalent postfix notation ? (Hint: First convert the infix expression into its equivalent binary tree representation) CS 307 Fundamentals of Computer Science

CS 307 Fundamentals of Computer Science

Algorithm buildExpressionTree (E): Input: Fully-parenthesized arithmetic Expression Output: A binary Tree T representing Expression S a new empty stack For i 0 to n-1 do { if E[i] is a variable or an operator then T  new binary tree with E[i] as root S.push(T) else if E[i] = “(“ then continue; else if E[i] = “)” T2  S.pop() T  S.pop() T1  S.pop() Attach T1 as T’s left subtree and T2 as its right subtree } Return S.pop() CS 307 Fundamentals of Computer Science

Algorithm PostFix (E): Input: Full-Parenthesized arithmetic Expression ,E. Output: PostFix notation of E T  buildExpressionTree (E) Postorder-Traversal (T.root) // this gives the Postfix notation of the expression Preorder-Traversal (T.root) // this gives the Prefix notation of the expression Inorder-Traversal (T.root) // this gives the Infix notation of the expression CS 307 Fundamentals of Computer Science

Representing Graphs Assume V = {1, 2, …, n} An adjacency matrix represents the graph as a n x n matrix A: A[i, j] = 1 if edge (i, j)  E (or weight of edge) = 0 if edge (i, j)  E CS 307 Fundamentals of Computer Science

Graphs: Adjacency Matrix Example: A 1 2 3 4 1 a 2 d 4 b c 3 How much storage does the adjacency matrix require? A: O(V2) CS 307 Fundamentals of Computer Science

Graphs: Adjacency List Adjacency list: for each vertex v  V, store a list of vertices adjacent to v Example: Adj[1] = {2,3} Adj[2] = {3} Adj[3] = {} Adj[4] = {3} Variation: can also keep a list of edges coming into vertex 1 2 4 3 Adjacency lists take O(V+E) storage CS 307 Fundamentals of Computer Science

Graph Problems Given an adjacency-list representation of a directed graph, how long does it take to compute the out-degree and in-degree of every vertex ? Also, how long is it going to take if we use the adjacency matrix instead ? CS 307 Fundamentals of Computer Science

Solution Given the adjacency-Matrix representation of a graph the out and in-degree of every node can easily be computed as follows. For I  1 to v // v vertices For J  1 to v // v vertices if(A[I][J] == 1) then outdegree[I]++; indegree[J]++; Time complexity : O(V2) CS 307 Fundamentals of Computer Science

Solution Given the adjacency-List representation of a graph the out and in-degree of every node can easily be computed as follows. For I  1 to v // n vertices For J  1 to A[I].size() // neighbours of vertex I n  A[i].get(J) outdegree[I]++; indegree[n]++; Time complexity : O(V+E) CS 307 Fundamentals of Computer Science

Problem The transpose of a directed graph G = (V,E) is the graph G with all its edges reversed. Describe efficient algorithms for computing the transpose of G for both adjacency list and matrix representations. What are the running times ? CS 307 Fundamentals of Computer Science

Solution Given the adjacency-Matrix representation of a graph the transpose can be computed as follows. For I  1 to v // v vertices For J  1 to v // v vertices Transpose[I][J] = A[J][I] Time complexity : O(V2) CS 307 Fundamentals of Computer Science

Solution Given the adjacency-List representation of a graph the transpose can be computed as follows. For I  1 to n // n vertices For J  1 to A[I].size() // neighbours of vertex I Transpose[I].add(A[J].get(I)) Time complexity : O(V+E) CS 307 Fundamentals of Computer Science

Predecessor and Successor in a BST Successor of node x is the node y such that key[y] is the smallest key greater than key[x]. The successor of the largest key is NIL. Search consists of two cases. If node x has a non-empty right subtree, then x’s successor is the minimum in the right subtree of x. If node x has an empty right subtree, then: As long as we move to the left up the tree (move up through right children), we are visiting smaller keys. x’s successor y is the node that x is the predecessor of (x is the maximum in y’s left subtree). In other words, x’s successor y, is the lowest ancestor of x whose left child is also an ancestor of x. CS 307 Fundamentals of Computer Science

Pseudo-code for Successor Tree-Successor(x) if right[x]  NIL 2. then return Tree-Minimum(right[x]) 3. y  p[x] 4. while y  NIL and x = right[y] 5. do x  y 6. y  p[y] 7. return y 56 26 200 18 28 190 213 12 24 27 Code for predecessor is symmetric. Running time: O(h) CS 307 Fundamentals of Computer Science

Properties of Binary Trees Note that external nodes are also called leaf nodes Let # leaf nodes = E, # internal nodes = I , Height = H, # nodes = N, E = I + 1 N = E + I (# nodes at level x) <= 2x E <= 2H H >= log2 I A full binary tree has (2H+1 – 1) nodes. (H+1) ≤ E ≤ 2H. H≤ I ≤ 2H -1. 2H+1 ≤ N ≤ 2H -1. log2(N+1)-1 ≤ H ≤ (N-1)/2 log2(E) ≤ H ≤ E-1 log2(I+1) ≤ H ≤ I CS 307 Fundamentals of Computer Science