Introduction to Algorithms
December 8, 2004
Massachusetts Institute of Technology
6.046J/18.410J
Professors Piotr Indyk and Charles E. Leiserson
Handout 34
Problem Set 9 Solutions
Reading:
Chapters 32.1–32.2, 30.1–30.2, 34.1–34.2, 35.1
Both exercises and problems should be solved, but
only the problems
should be turned in.
Exercises are intended to help you master the course material. Even though you should not turn in
the exercise solutions, you are responsible for material covered in the exercises.
Mark the top of each sheet with your name, the course number, the problem number, your
recitation section, the date and the names of any students with whom you collaborated.
Threehole punch your paper on submissions.
You will often be called upon to “give an algorithm” to solve a certain problem. Your writeup
should take the form of a short essay. A topic paragraph should summarize the problem you are
solving and what your results are. The body of the essay should provide the following:
1. A description of the algorithm in English and, if helpful, pseudocode.
2. At least one worked example or diagram to show more precisely how your algorithm works.
3. A proof (or indication) of the correctness of the algorithm.
4. An analysis of the running time of the algorithm.
Remember, your goal is to communicate. Full credit will be given only to correct algorithms
that are
which are described clearly
. Convoluted and obtuse descriptions will receive low marks.
Exercise 91.
Online String Matching
Recall that in an online algorithm, the input is generated as the algorithm is running. The idea
is to solve the problem efficiently before seeing all the input. You can’t scan forward to look at
’future’ input, but you can store all input seen so far, or some computation on it.
(a)
In this setting, the text
T
[1
. . . n
]
is being broadcast on the network, one letter at a
time, in the order
T
[1]
, T
[2]
, . . .
. You are interested in checking if the text seen so far
contains a pattern
P
, where
P
has length
m
. Every time you see the next letter of the
text
T
, you want to check if the text seen so far contains
P
.
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 Fall '08
 ErikDemaine
 Algorithms, Greedy algorithm, KR

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