t2keysp01 - COP3530.01, Spring 2001 S. Lang April 05, 2001...

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COP3530.01, Spring 2001 April 05, 2001 S. Lang Solution Key to Test #2 1. (a) (10 pts.) Construct the Huffman codes for 4 characters A, B, C, and D, assuming their frequencies are 28, 30, 10, and 24, respectively. A (28) ----- (58) ----- (92) Huffman’s codes: A – 00; B – 01; C – 10; D – 11. (The merge tree is represented with the leaf nodes on the B (30) -------- left side of the figure, the left branch on top of the right Branch for each internal node) C (10) ----- (34) -------- D (24) -------- (b) (5 pts.) If we are constructing the Huffman codes for n characters given their frequencies, give the time complexity of the algorithm with a brief (two lines) explanation. It takes O( n ) time to convert the array of n frequency values into a min-heap. Then we repeatedly delete the next two smallest numbers from the heap, add then insert the sum into the heap, representing an internal node. This process continues until a single value is left. The loop takes O( n ) iterations; each iteration takes O(lg n ) time, so the total time of the loop is O( n lg n ). The codes can be generated by a tree traversal for O( n ) time. The algorithm’s total time is O( n lg n ). 2. (15 pts.) Suppose a binary min-heap is maintained in an integer array H [1. . n ]. If the value H [ k ] is modified to become smaller , 1 k n , write in C++ or Java ( circle which one you choose ) a function that restores the heap property using the following function prototype: void percolateUp (int k ) // assume H [1. . n ] and n “global” (See Text for the answer.) 3. Suppose we use a divide-and-conquer strategy to solve a problem that uses an array of size n as input. (i) Use bn amount of time to divide the problem into two approximately equal- sized subproblems. (ii) Solve the two subproblems separately using recursion; the recursion terminates when n = 1. (iii) Use cn 2 amount of time to combine the two solutions of Part (ii) into a solution to the original problem. (a)
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t2keysp01 - COP3530.01, Spring 2001 S. Lang April 05, 2001...

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