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1. For a directed graph, the underlying undirected graph is a graph that is obtained by ignoring the direction

of the edges or replacing each directed edge by an undirected edge. We defined a tournament graph to be

a directed graph that its underlying undirected graph is a complete graph (i.e. if we ignoring the direction

there is an edge between every pair of nodes).

(a) A king in a graph is a node that can reach any other node by a path of length 2. Prove that there is

a king in the tournament graph.

(b) Design an algorithm that finds a king in any tournament graph. Your algorithm should be linear time

in the number of edges.

2. Given a real number X and two sets S1 and S2 that contain some real numbers.

(a) Design an O(n log n) algorithm to find whether two sets contain a same number, where n is the total

number of elements.

(b) Design an O(n log n) algorithm to find whether there exists a number from S1, and an number from

S2, whose sum is exactly X. (Hint: reduce the problem to the above problem)

(c) Given an input set S containing n real numbers, and a real number X.

i. Design an algorithm to determine whether there are two elements of S whose sum is exactly X.

The algorithm should run in time O(n log n).

ii. Suppose now that the set S is given in a sorted order. Design an algorithm to solve this problem

in time O(n).

3. Given a n ⇥ n binary array. Each row represents a binary number, and so there are n numbers in the array.

Assume one operation is looking up a bit in the array and the time is measured by the number of such

operations.

(a) Design an O(n) time algorithm to find a number that is different with all n numbers in the array.

(b) Prove that ⌦(n) is a lower bound on the number of steps required to solve this problem.

4. Modify radix sort to work for variable-length strings and w.l.o.g assume the numbers are given in base n,

and there are n numbers. You can no longer assume that all numbers have same digits. Some numbers may

be long and some short. It is possible of course to “pad” all numbers by adding “dummy” (0) digits to make

them all of the same length. Find an algorithm that avoids doing that and achieves a running time linear in

the total number of digits.

∆ Homework assignments are STRICTLY due on the exact time indicated. Please submit a hard copy of your

homework solution with your Name, Bruin ID, Discussion Number, clearly indicated on the first page.

If your homework consists of multiple pages, please staple them together. Email attachments or other

electronic delivery methods are not acceptable.

∆ We recommend to use LATEX, LYX or other word processing software for submitting the homework. This is

not a requirement but it helps us to grade the homework and give feedback. For grading, we will take into

account both the correctness and the clarity. Your answer are supposed to be in a simple and understandable

manner. Sloppy answers are expected to receiver fewer points.

∆ Unless specified, you should justify your algorithm with proof of correctness and time complexity.

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