课程名称︰资料结构与演算法下 课程性质︰系必修 课程教师︰林轩田 开课学院：电资学院 开课系所︰资讯系 考试日期（年月日）︰2011/04/24 考试时限（分钟）：150 是否需发放奖励金：是 （如未明确表示，则不予发放） 试题 : This is a open-book exam. You can use any printed materials as your reference during the exam. Any electronic devices are not allowed. Any form of cheating, lying or plagiarism will not be tolerated. Students can get zero scores and/or get negative scores and/or fail the class and/or be kicked out of school and/or receive other punishments for those kinds of misconducts. Bot English and Chinese (if suited) are allowed for answering the question. We do not accept any other languages. Thank you for coming to the DSA Night. The night is prepared with the help of our eight divisions: Organize, Budget, Entrace, Dance, Drama, Music, Video and Band. Each division includes a few memebers (questions). There are 20 questions, each worth 10 points - the full credit is 200 points. For the 20 questions, 8 of them are marked with * and are supposedly simply; 8 of them are marked with ** and are supposedly regular; 4 of them are marked with *** and are supposedly difficult. 1. Organize: Post-order or In-order? (1) (10%, *) Consider a full binary tree of depth 4 with nodes numbered from 1 to 15 such that node i has a left child 2i and a right child 2i+1. Do a post-order traversal in the full binary tree above and print out the node numbers visited. (2) (10%, **) Assue that the in-order traversal of one binary tree to be GDHBIELJMACKF and its post-order traversal to be GDHILMJEBKFCA. Please draw the binary tree. 2. Budget: Expression or Unlimited? (1) (10%, *) Draw the expression tree of the following infix expression with usual precedence and left association. Note that the tree should contain only the binary operators and the operands. (2) (10%, *) What is the prefix notation of the expression above? 3. Entrance: Queue or Stack? (1) (10%, *) Considering representing two stacks with one array of size 6 by putting the bottoms of the stacks in the two ends of the array. List the contents of the array after doing all of the following operations orderly. For locations without contents, please use a special character to represent them. push(stack 1, 5); push(stack 1, 5); push(stack 2, 6); push(stack 1, 6); push(stack 2, 3); pop(stack 1); push(stack 2, 5); pop(stack 1); (2) (10%, **) Write down an algorithm that uses one queue (with enqueue and dequeue operations) and at most O(1) of additional memory to simulate one stack. 4. Dance: Balanced or not? Consider an ordered array a of N nonzero integers a_0 <= a_1 <= a_2 <= ... <= a_(N-1). Define the unbalancedness of a to be (number of positive elements in a) - (number of negative elements in a). (1) (10%, **) Write down an O(logN)-time algorithm that computes the unbalanced-ness of a. Briefly describe why the algorithm is correct. (Hint: think about binary search.) (2) (10%, ***) Consider only the cases with N = 2^k, rigorously prove that your algorithm is indeed O(log N) = O(k)-time by either a mathematical induction on k or any other proof of your choice. 5. Drama: Complex or not? For this question, you can only use the definition of O(.) in the textbook: f(n) = O(g(n)) if and only if there exist positive constants c and n_0 such that f(n) <= c*g(n) for all n such that n >= n_0. The lg means log2 here. (1) (10%, *) Prove that n = O(2^n). (2) (10%, **) If f(n) = O(g(n)) and f(n) > 0; g(n) >= 2 for n >= 1. Prove that lg f(n) = O(lg g(n)). (3) (10%, *) Prove that lg n = O(n). (Hint: check results above.) (4) (10%, ***) Consider some f(n) and g(n) such that lg f(n) = O(lg g(n)) and g(n) >= 1 for n >= 1. Construct a counter-example to disprove that f(n) = O(g(n)). 6. Music: KMPlayer or not? The key idea is the KMP string matching algorithm is the failure function of a pattern p_0p_1...p_(n-1). f_p(j) = largest i < j such that p_0p_1...p_i = p_(j-i)p_(j-i+1)...p_j if such i >= 0 exists; -1 otherwise. (1) (10%, **) Given a failure function f_p for a pattern p with length 8 and consider q = p_2...p_(n-1) (the tail of p) with length 6. Consider the following f_p. j 0 1 2 3 4 5 6 7 f_p(j) -1 -1 0 1 2 0 1 -1 What is f_q? Briefly justify your answers. (2) (10%, **) Consider the same f_p above. Assume that we know, additionally, some elements of p. j 0 1 2 3 4 5 6 7 p a b d f_p(j) -1 -1 0 1 2 0 1 -1 What is the full pattern string p? Briefly justify your answers. 7. Video: Compress or not? Run-length encoding is a very common way of data compression. Consider a vector a = (0,1,1,1,3,3,3,3,5,5,6,6,1). It's run-length encoding is an array of pairs like [(1,0),(3,1),(4,3),(2,5),(2,6),(1,1)], which means there is one "0", followed by three "1", followed by four "3", followed by two "5", followed by two "6", followed by one "1". (1) (10%, *) Given the run-length encoding (as a lengt-M dense array of pairs) of a vector a of length N, write down an algorithm that computes N-1 its average (1/N) * Sigma(a_i) i=0 (2) (10%, **) Given the run-length encoding of two vectors a and b, both of length N, write down an algorithm that computes their inner product N-1 Sigma(a_i * b_i). i=0 8 Band: Factorize or not? Decomposing a positive integer into multiplications of prime integer factors (so-called factorization) is important in many areas. For instance, 12 = 2*2*3. Assume that there are N integers 1, 2, ..., N. One way to represent the factorization is to maintain a linked list of those factors for every integer. For instance, 12 would be associated with a linked list (2,2,3) and 6 would be associated with a linked list (2,3). As you can see, there is a waste in memory because the tail part of the linked list of 12 is essentially the same as the linked list of 6. Dr. Fact thought of a data structure to eliminate the memory waste. He lets N link to M if and only if N/M is the smallest prime factor of N. So for instance, 12 links to 6, which links to 3, which links to 1, which links to nothing. Thus, starting from 12, we can get all its prime factors by moving in the order of 12, 6 ,3 ,1. The links for integers 1 to 12 are follows. 1->nothing; 2->1; 3->1; 4->2; 5->1; 6->3; 7->1; 8->4; 9->3; 10->5; 11->1; 12->6; We will represent the links within an array next where next[i] stores the integer that i links to. (1) (10%, *) Given the next array of size N, write down an algorithm taht determines whether a given integer X between 2 and N is a prime. (2) (10%, **) The following algorithm computes the value of next[X] for a given X. Compute-Next(integer X) for i <- 2 to ... do if X mod i = 0 them return X/i end if end for return 1 Describe the tightest upper bound for i to make the algorithm correct and briefly justify you answer. (3) (10%, ***) For X = p_1^n_1 * p_2^n_2...p_k^n_k where p_i are primes, its number of positive divisors is (n_1 + 1) * (n_2 + 1) * ... * (n_k + 1). (4) (10%, ***) Given the next array of size N and two integers X, Y that are both between 2 and N. Write down an algorithm that print out the links from X * Y (which can be larger than N) to 1. For instance, if N = 20, X = 6, Y = 10, the algorithm should print out 60->30->15->5->1. --

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