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Unformatted text preview: recurrence equation that is more challenging to solve than this one. Thus, it is useful to develop some general ways of solving
the kinds of recurrence equations that arise in the analysis of divideandconquer
algorithms. Chapter 5. Fundamental Techniques 264 The Iterative Substitution Method
One way to solve a divideandconquer recurrence equation is to use the iterative
substitution method, which is more colloquially known as the “plugandchug”
method. In using this method, we assume that the problem size n is fairly large
and we then substitute the general form of the recurrence for each occurrence of
the function T on the righthand side. For example, performing such a substitution
with the mergesort recurrence equation yields the equation
T (n) = 2(2T (n/22 ) + b(n/2)) + bn
= 22 T (n/22 ) + 2bn.
Plugging the general equation for T in again yields the equation
T (n) = 22 (2T (n/23 ) + b(n/22 )) + 2bn
= 23 T (n/23 ) + 3bn.
The hope in applying the iterative substitution method is that, at some point, we
will see a pattern that can be converted into a general closedform equation (with
T only appearing on the lefthand side). In the case of the mergesort recurrence
equation, the general form is
T (n) = 2i T (n/2i ) + ibn.
Note that the general form of this equation shifts to the base case, T (n) = b, when
n = 2i , that is, when i = log n, which implies
T (n) = bn + bn log n.
In other words, T (n) is O(n log n). In a general application of the iterative substitution technique, we hope that we can determine a general pattern for T (n) and that
we can also ﬁgure out when the general form of T (n) shifts to the base case.
From a mathematical point of view, there is one point in the use of the iterative
substitution technique that involves a bit of a logical “jump.” This jump occurs at
the point where we try to characterize the general pattern emerging from a sequence
of substitutions. Often, as was the case with the mergesort recurrence equation,
this jump is quite reasonable. Other times, however, it may not be so obvious what
a general form for the equation should look like. In these cases, the jump may
be more dangerous. To be completely safe in making such a jump, we must fully
justify the general form of the equation, possibly using induction. Combined with
such a justiﬁcation, the iterative substitution method is completely correct and an
often useful way of characterizing recurrence equations. By the way, the colloquialism “plugandchug,” used to describe the iterative substitution method, comes
from the way this method involves “plugging” in the recursive part of an equation
for T (n) and then often “chugging” through a considerable amount of algebra in
order to get this equation into a form where we can infer a general pattern. 5.2. DivideandConquer 265 The Recursion Tree
Another way of characterizing recurrence equations is to use the recursion tree
method. Like the iterative substitution method, this technique uses repeated substitution to solve a recurrence equation, but it differs from the iterative substitution
method in that, rather than being an algebraic approach, it is a visual approach. In
using the recursion tree method, we draw a tree R where each node represents a
different substitution of the recurrence equation. Thus, each node in R has a value
of the argument n of the function T (n) associated with it. In addition, we associate
an overhead with each node v in R, deﬁned as the value of the nonrecursive part
of the recurrence equation for v. For divideandconquer recurrences, the overhead
corresponds to the running time needed to merge the subproblem solutions coming
from the children of v. The recurrence equation is then solved by summing the
overheads associated with all the nodes of R. This is commonly done by ﬁrst summing values across the levels of R and then summing up these partial sums for all
the levels of R.
Example 5.3: Consider the following recurrence equation:
T (n) = if n < 3
b
3T (n/3) + bn if n ≥ 3. This is the recurrence equation that we get, for example, by modifying the mergesort algorithm so that we divide an unsorted sequence into three equalsized sequences, recursively sort each one, and then do a threeway merge of three sorted
sequences to produce a sorted version of the original sequence. In the recursion tree
R for this recurrence, each internal node v has three children and has a size and an
overhead associated with it, which corresponds to the time needed to merge the subproblem solutions produced by v’s children. We illustrate the tree R in Figure 5.4.
Note that the overheads of the nodes of each level sum to bn. Thus, observing that
the depth of R is log3 n, we have that T (n) is O(n log n).
Overhead
bn bn bn Figure 5.4: The recursion tree R used in Example 5.3, where we show the cumula tive overhead of each level. Chapter 5. Fundamental Techniques 266 The GuessandTest Method
Another method for solving recurrence equations is the guessandtest technique.
This technique involves ﬁrst making an educated guess as to...
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 Spring '14

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