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This fle contains the exercises, hints, and solutions For Chapter 9 oF the
book ”Introduction to the Design and Analysis oF Algorithms,” 2nd edition, by
A. Levitin. The problems that might be challenging For at least some students
are marked by
±
;
those that might be diﬃcult For a majority oF students are
marked by
²
.
Exercises 9.1
1. Give an instance oF the changemaking problem For which the greedy al
gorithm does not yield an optimal solution.
2. Write a pseudocode oF the greedy algorithm For the changemaking prob
lem, with an amount
n
and coin denominations
d
1
>d
2
>.
.
.>d
m
as its
input. What is the time eﬃciency class oF your algorithm?
3. Consider the problem oF scheduling
n
jobs oF known durations
t
1
,...,t
n
For
execution by a single processor. The jobs can be executed in any order,
one job at a time. You want to fnd a schedule that minimizes the total
time spent by all the jobs in the system. (The time spent by one job in
the system is the sum oF the time spent by this job in waiting plus the
time spent on its execution.)
Design a greedy algorithm For this problem.
±
Does the greedy algo
rithm always yield an optimal solution?
4. Design a greedy algorithm For the assignment problem (see Section 3.4).
Does your greedy algorithm always yield an optimal solution?
5.
Bridge crossing revisited
Consider the generalization oF the bridge cross
ing puzzle (Problem 2 in Exercises 1.2) in which we have
n>
1
people
whose bridge crossing times are
t
1
,t
2
n
.
All the other conditions oF
the problem remain the same: at most two people at the time can cross
thebr
idge(andtheymovew
iththespeedo
Fthes
lowero
Fthetwo)and
they must carry with them the only ﬂashlight the group has.
Design a greedy algorithm For this problem and fnd how long it will
take to cross the bridge by using this algorithm.
Does your algorithm
yields a minimum crossing time For every instance oF the problem? IF it
does–prove it, iF it does not–fnd an instance with the smallest number
oF people For which this happens.
6.
BachetFibonacci weighing problem
±ind an optimal set oF
n
weights
{
w
1
,w
2
,...,w
n
}
so that it would be possible to weigh on a balance scale
any integer load in the largest possible range From 1 to
W
, provided
a.
±
weights can be put only on the Free cup oF the scale.
b.
²
weights can be put on both cups oF the scale.
1
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View Full Document7. a. Apply Prim’s algorithm to the following graph. Include in the priority
queue all the vertices not already in the tree.
c
a
b
d
5
4
7
6
e
23
45
b. Apply Prim’s algorithm to the following graph. Include in the priority
queue only the fringe vertices (the vertices not in the current tree which
are adjacent to at least one tree vertex).
i
e
f
j
9
5
h
d
g
c
b
a
l
k
3
6
8
6
5
4
3
6
3
2
1
2
5
4
75
43
8. The notion of a minimum spanning tree is applicable to a connected
weighted graph. Do we have to check a graph’s connectivity before ap
plying Prim’s algorithm or can the algorithm do it by itself?
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 Spring '10
 DrT
 Algorithms

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