Purdue University CS 381
Homework 3 (100 points)
Problem 1 (30 points). An axis-aligned rectangle is a rectangle with edges parallel to the coordinate axes. Given n points with their x and y coordinate values, design an O(n log n) time Divide-and-Conquer algorithm which finds an axis-aligned rectangle with minimum perimeter that covers at least 3 points out of the n given points.
In the following example, n = 5, a = (2,3), b = (3,2), c = (4,4), d = (6,1), and e = (7,2). The minimum perimeter axis-aligned rectangle R covers a, b, and c. The vertices of R are (2, 2), (2, 4), (4, 2), and (4, 4).
y
4 3 2 1
set up in a way such that:
• You may throw a consecutive sequence of balls into the same bin.
• Once you move on from bin i, you cannot come back to throw later balls into it.
Define the load of a bin as the total weight of all balls in it. Design and analyze an efficient DP algorithm to determine the balls contributing to the load on every bin so as to minimize the maximum load over all bins. In your solution, make sure to explicitly state:
1. the optimal substructure and why it is optimal,
2. the DP recurrence formula including base cases,
3. whether your algorithm is bottom-up or top-down DP, 4. the description of your algorithm, and
5. the time and space complexity of your algorithm.
For example, suppose B = [20, 9, 15, 10, 32] are the weights of the balls and k = 3, then your algorithm should return l1 = [20, 9], l2 = [15, 10], and l3 = [32], where li represents the list of balls corresponding to the load on bin i. The minimum max load is 32 for the given input B, and there are no other ways to throw the balls differently so that the minimum max load over all bins is any lower. In case of multiple optimal solutions, your algorithm may output any one of them.
c
a
b
e d
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x
You are given k distinct bins arranged in an ordered sequence. Your task is to throw n balls (in order) into these k bins. However, every ball has an associated weight with it. The task is
Problem 2 (30 points).
R
Problem 3 (40 points). You and your friend decide to play a game of Connect 41. After you set up your friend in a trap, they lose the game and get frustrated. In their frustration they propose the following “Disconnect” game to you. The game “Disconnect” is played on the same board as a Connect 4 game but does not have the same rules. Specifically, on a filled-up Disconnect board of size n × 7, you and your opponent will alternate removing pieces from the board and collecting points for them. The “filled-up” board is not necessarily in an end-game position of Connect 4. It could have more than 4 pieces lined up or less than 4 lined up for either color. Also, the number of red pieces may not necessarily equal the number of blue pieces. The player with the most points in the end wins. The rules of “Disconnect” are as follows:
1. A player can remove any piece from the board, G, if and only if no other pieces sit on top of it. Specifically piece Gi,j can be removed if and only if pieces Gk,j have been removed for all k > i or if i = n.
2. If a player removes a piece Gi,j of their own color, then they lose 1 point and their opponent gains 1 point.
3. If a player removes a piece Gi,j of the opponent’s color, then they gain k points, where k is equivalent to the length of the longest connection that piece Gi,j is a part of at that current state, and their opponent loses k points.
A connection is defined as a contiguous straight-line sequence of same-colored pieces (diagonal, horizontal, or vertical). For instance in Figure 2, the blue piece Gn,2 is a part of two connections: {Gn,2,Gn−1,3,Gn−2,4} and {Gn,2,Gn,3}. The former being of length 3 and the latter being of length 2.
Figure 1: Example Board Game State for “Disconnect”
Top of Game Board
n,1
n,2
n,3
n,4
n,5
n,6
n,7
n−1, 1
n−1, 2
n−1, 3
n−1, 4
n−1, 5
n−1, 6
n−1, 7
n−2, 1
n−2, 2
n−2, 3
n−2, 4
n−2, 5
n−2, 6
n−2, 7
2,1
2,2
2,3
2,4
2,5
2,6
2,7
1,1
1,2
1,3
1,4
1,5
1,6
1,7
Bottom of Game Board
To give a few more examples of points, consider the configuration in Figure 2, where a blank space represents a piece that has been removed in a prior turn. If it is Red’s turn, then removing piece:
• Gn,1 would result in -1 points for Red and +1 points for Blue. 1For this problem, you only need to read the first paragraph of the linked article.
• Gn,2 would result in +3 points for Red and -3 points for Blue.
• Gn,3 would result in +2 points for Red and -2 points for Blue.
• Gn−1,4, Gn−2,5, or Gn−1,6 would result in -1 points for Red and +1 points for Blue. • Gn,7 would result in +2 points for Red and -2 points for Blue.
Your goal is to create an optimal DP solution for Disconnect 4. Specifically, describe and analyze a polynomial-time Dynamic Programming algorithm to determine your optimal final score under the assump- tion that your opponent also plays optimally. You can assume you are the Red player and you will always go first. In your solution, make sure to explicitly state:
1. the optimal substructure and why it is optimal,
2. the DP recurrence formula including base cases,
3. whether your algorithm is bottom-up or top-down DP, 4. the description of your algorithm, and
5. the time and space complexity of your algorithm.
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