課程名稱︰演算法設計與分析
課程性質︰資訊系必修
課程教師︰蕭旭君
開課學院：電機資訊學院
開課系所︰資訊工程學系
考試日期（年月日）︰2014/11/14
考試時限（分鐘）：180
是否需發放獎勵金：是
（如未明確表示，則不予發放）
試題 :
Instructions
・This is a 3-hour close-book exam.
・There are 7 questions worth of 100 points plus 30 bonus points. The maximum
of your score is 100, so you can allocate your time and select problems based
on certain optimal strategies of your own.
・For any of the questions or sub-questions except bonus ones, you get 1/5 of
the credit when leaving it blank and get 0 when the answer is completely wrong.
You get partial credit when the answer is partially correct.
・Please write clearly and precisely; avoid giving irrelevant information.
・You have access to the appendix on the last page.
・Please print your name, student ID, and page number on every answer page.
Problem 1 Short Answer Question (30 points)
Answer the following questions and briefly justify your answer. For (a)-(d),
decide whether you think the statement, particularly the underlined phrase, is
true or false. Briefly explain your choice to receive full credit.
(a) (3 points) True or False: To prove the correctness of a greedy algorithm,
we must prove that __every optimal solution contains our greedy choice__.
(b) (3 points) True or False: When items have __non-integer weights__, it may
be extremely inefficient to solve the 0/1 Knapsack Problem using dynamic
programming because of __lack of overlapping subproblems__.
(c) (3 points) True or False: (m_1,w_3),(m_2,w_2),(m_3,w_1) is one stable
matching in the following preference lists:
┌──┬───────┐ ┌──┬───────┐
│ │ 1st 2nd 3rd│ │ │ 1st 2nd 3rd│
├──┼───────┤ ├──┼───────┤
│ m_1│ w_3 w_2 w_1│ │ w_1│ m_1 m_2 m_3│
│ m_2│ w_2 w_1 w_3│ │ w_2│ m_1 m_2 m_3│
│ m_3│ w_3 w_2 w_1│ │ w_3│ m_2 m_1 m_3│
└──┴───────┘ └──┴───────┘
(d) (3 points) True or False: If f(n) is O(g(n)), then 2^f(n) is O(2^g(n)).
(e) (3 points) Given the recurrence relation A(i,j) = F(A(i,j-1),A(i-1,j-1),
A(i-1,j+1)), provide a valid traversal order to fill the DP table or justify
why no valid traversal exists.
(f) (3 points) Given the recurrence relation A(i,j) = F(A(i-1,j-1),A(i+1,j+1)),
provide a valid traversla order to fill the DP table or justify why no valid
traversal exists.
(g) (3 points) Suppose you have designed a Divide and Conquer algorithm whose
running time T(n) can be expressed by T(n) = aT(n/b)+f(n). Briefly explain
what a,b, and f(n) represent in your algorithm.
(h) (3 points) Solve the following recurrence by giving a tight Θ-notation
bound:
T(n) = 2T(n/2)+(n^2) log n
(i) (6 points) Describe a real-world problem that you have solved or have
thought about solving using technique learned in the class. Make an educated
guess about how fast your algorithm might be, and how much time it would take
to solve the same problem using a brute-force approach instead.
Problem 2: nth Smallest in Two Databases (15 points)
You're given two databases D_1 and D_2, each of which contains n numbers. These
2n numbers are all distinct. For each database, you can query for the kth
smallest number for any 1≦k≦n. You want to find the nth smallest number among
these 2n numbers.
(a) (10 points) Please design a Divide-and-Conquer algorithm to find the nth
smallest number among these 2n numbers in O(log n) queries.
Hint: Let m = \floor{n/2} and let v_1,m and v_2,m be the mth smallest
number in database D_1 and D_2, respectively. Let x be the nth smallest number
among these 2n numbers. Consider where x is in three cases: v_1,m > v_2,m,
v_1,m = v_2,m, v_1,m < v_2,m.
(b) (5 points) Briefly justify the running time of your algorithm is indeed
O(log N) queries.
Problem 3: Longest Path (20 points)
(a) (3 points) Briefly explain what optimal substructure is.
(b) (5 points) Consider a graph G = (V,E), a start node s ∈ V, and end node
t ∈ V, and weight w_e of each edge e ∈ E. The Longest Path Problem is to find
a longest simple path going from s to t on this graph. (The length of a path is
the sum of weights of all edges on the path. A simple path is a path that
visits a node at most once, that is, a path without cycles.) Please construct a
counterexample to show that the Longest Path Problem has no optimal
substructure.
(c) Your friend made a interesting observation that the Longest Path Problem on
certain types of graphs does exibit optimal substructure. To test your friend's
conjecture, you decide to use a dynamic programming algorithm to find one
longest path from top-left to bottom-right on a grid-like directed graph with
edges going downward and rightward only. As an example, Figure 1 shows a 4-by-4
grid in which each edge is labeled with a weight.
Answer the following questions:
1. (3 points) Find one longest path in the graph in Figure 1.
2. (6 points) Consider a n-by-n grid. Suppose you want to find the length
of a longest path using dynamic programming. Please define your
subproblems and represent the value of the optimal solution using a
recurrence.
3. (3 points) What's special about this type of graphs? Please explain why
the Longest Path Problem has no optimal substructure in general but has
optimal substructure in this special type of graphs. An informal
observation would be sufficient.
3 4 5
ｓ→→→○→→→○→→→○
8↓ 5↓ 5↓ 8↓
↓ 8 ↓ 7 ↓ 3 ↓
○→→→○→→→○→→→○
3↓ 6↓ 5↓ 4↓
↓ 3 ↓ 9 ↓ 7 ↓
○→→→○→→→○→→→○
7↓ 5↓ 3↓ 1↓
↓ 3 ↓ 1 ↓ 4 ↓
○→→→○→→→○→→→ｔ
Figure 1: Find one longest path from s to t.
Problem 4: The Rise of the Planet of the Apes (15 points)
Being a brilliant ape, Caesar is learning a sign language to communicate with
his trainer, Will. In this sign language, each word, is represented by a
sequence of gestures. This sign language should be prefix-free, meaning that no
word is a prefix of another word, since Caesar is smart enough to combine words
into phrases. Will wants to teach Caesar the following seven words that are
frequently used in their daily activities:
┌─────┬─────────────────────┐
│ Word │food sleep play drink open who come │
├─────┼─────────────────────┤
│ Frequency│0.25 0.15 0.12 0.19 0.07 0.14 0.08 │
└─────┴─────────────────────┘
(a) (5 points) Will knows that Caesar can do two gestures─pointing his index
finger upward (UP) pointing his index finger downward (DOWN)─without
difficulty. Please help Will to create a prefix-free binary sign language so as
to minimize the average number of gestures per word. This sign language should
contain the above seven words and each word is represented by a sequence of UP
and DOWN gestures.
(b) (5 points) Caesar now can do three gestures─UP, DOWN, and WAVE. Please
create a prefix-free ternary sign language so as to minimize the average
number of gestures per word. The sign language should contain the above seven
words and each word is represented by a sequence of UP, DOWN, and WAVE gesture.
(c) (5 points) Following (a), Will observes that the UP gesture consumes three
times of energy than the DOWN gesture. Will comes up with greedy heuristic in
the hope to minimize the average energy consumption per word. His greedy
heuristic is as follows. Given n words and their frequencies, Will first draws
a binary prefix tree that minimizes the average number of gestures per word in
its corresponding binary sign language. He then re-labels each edge such that
for every node v and its left node v_l and right node v_r, edge (v,v_l) is
re-labeled with UP and edge (v,v_r) is re-labeled with DOWN if the sum of the
frequencies of the leaf nodes in v's left subtree is lower than it of its right
subtree. The labels are switched otherwise. Please show that this greedy
algorithm is incorrect. You don't need to use the same input (i.e., the 7 words
and their frequency distributions) in your counterexample.
Problem 5: Counting Inversions (10 points + 10 bonus points)
(a) Counting inversions revisited (10 points) Given a sequence of unique
numbers B = b_1,b_2,...b_n, an inversion is a pair of numbers b_i and b_j in
this sequence such that i < j and b_i > b_j. Let I(B) be the set of inversions
in B. For example, if B = <1, 3, 5, 2>, I(B) = {(3,2),(5,2)}, and the number of
inversions |I(B)| is 2.
Given an unsorted sequence B with n numbers, please design an efficient
algorithm to calculate the number of inversions |I(B)| in O(n log n) time.
Please briefly explain why your algorithm indeed runs in O(n log n).
(b) Bonus: Counting significant inversions (10 points) Given a sequence of
unique numbers B = b_1,b_2,...b_n, a significant inversion is a pair of numbers
b_i and b_j in this sequence such that i < j and b_i > 2b_j. Let SI(B) be the
set of significant inversions in B. For example, if B = <1, 3, 5, 2>,
SI(B) = {(5,2)}, and the number of significant inversions |SI(B)| is 1.
Given an unsorted sequence B with n numbers, please describe how to modify
your algorithm in part (a) for calculating the number of significant inversions
|SI(B)| in O(n log n) time. Please briefly explain why your algorithm indeed
runs in O(n log n).
Problem 6: Finding closest pair of points (10 points + 10 bonus points)
(a) (10 points) There are n points in a two-dimensional space, and point i's
x-y coordinate is (x_i,y_i). However, in this bizarre world, the distance
between points i and j is defined as d(i,j) = min {|x_i-x_j|,|y_i-y_j|}, which
is not Euclidean distance.
Suppose you are given arrays X_n and Y_n containing these n points sorted
by their x-coordinates and y-coordinates, respectively. Please design a greedy
algorithm to find a closest pair of points in O(n) time, and explain why your
algorithm is correct. You can prove it by showing your algorithm has the greedy
choice property and optimal substructure. Alternatively, you can also use other
proof techniques as long as you can show that your solution is indeed optimal.
Hint: first consider a 1D case where the distance is defined as
d(i,j) = |x_i-x_j| and then extend your solution to 2D.
(b) Bonus (10 points) There are n points in a two-dimensional space, and point
i's x-y coordinate is (x_i,y_i). However, in this bizarre world, the distance
between points i and j is defined as d(i,j) = max{|x_i-x_j|,|y_i-y_j|}, which
is not Euclidean distance.
Please design an algorithm to find a closet pair of points in O(n log n)
time, and explain why your algorithm is correct.
Problem 7: Fair Division of Christmas Gifts (10 bonus points)
Christmas is approaching. You're helping Santa Clauses to distribute gifts to
children.
For ease of delivery, you are asked to divide n gifts into two groups such
that the weight difference of these two groups is minimized. The weight of each
gift is a postive integer. Please designe an algorithm to find an optimal
division minimizing the value difference. The algorithm should find the minimal
weight difference as well as the groupings in O(nS) time, where S is the total
weight of these n gifts. Briefly justify the correctness of your algorithm.
Hint: This problem can be converted into making one set as close to S/2 as
possible.
Appendix
Master Theorem. Let a ≧ 1 and b > 1 be constants, let f(n) be a function, and
let T(n) be defined on the nonnegative integers by the recurrence
T(n)=aT(n/b) + f(n),
where we interpret n/b to mean either \floor{n/b} or \ceiling{n/b}. Then T(n)
has the following asymptotic bounds:
1. If f(n) = O(n^(log_b a-ε) for some constant ε> 0, then
T(n) = Θ(n^(log_b a)).
2. If f(n) = Θ(n^(log_b a)), then T(n) = Θ(n^(log_b a)log n).
3. If f(n) = Ω(n^(log_b a+ε)) for some constant ε> 0 and if
af(n/b) ≦ cf(n) for some constant c < 1 and all sufficiently large n,
then T(n) = Θ(f(n)).
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