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\begin{center}{\Large\bf FUNDAMENTAL ALGORITHMS\\ OLD MIDTERM SOLUTIONS}\end{center}
\ben
\item (20) Give an algorithm {\tt HORSE} with the following property.
The input is two arrays $A[1\cdots N]$, $B[1\cdots N]$, both
arrays in increasing order. The output is an array $C[1\cdots (2N)]$
which has all the values of the arrays $A,B$ and is in increasing
order. How long does your algorithm take. (Brief reason please!)
\So This is the MERGE algorithm as given in class.
\item (20) Illustrate the operation of {\tt COUNTINGSORT} on the
array
\[ A = (2,1,2,1,0,0,0,1) \]
with $n=8$ and $k=2$.
Pictures and some well chosen words, please. (You do not need
every detailed step but you must make clear the main steps.)
\So You have an auxilliary array $C[0\cdots 2]$ which is initially
zero. For $i=1$ to $n=8$, running through $A$, you increment
$C[A[i]]$ by one. This gives a count on $C$: $(3,3,2)$. Now
for $j=1$ to $2$ ($k$) you set $C[j]\leftarrow C[j]+C[j-1]$. This
turns $C$ into a {\em cumulative} count: $(3,6,8)$. Finally for
$i=8$ down to $1$ (going down the array $A$) you set (say)
$value=A[i]$ and then $position = C[value]$. Then you place
the value
$A[i]$ in that position in $B$: $B[position] \leftarrow value$.
Critically, you then decrement $C[value]$ so that next time you
hit that value it will be going to a different place.
\item (20) For the following algorithms let $T(N)$ denote the total number of times the
step after the
{\tt WHILE} step is reached.
For the first
algorithm give (five points) an {\em exact} formula for $T(N)$. For the second algorithm first
(ten points) give $T(N)$ as a precise sum. Then (five points)
Find $T(N)$ is the form $T(N)=\Theta(g(N))$ for a standard $g(N)$.
Reasons please!
\ben
\item
X=1
\\ WHILE X $<$ N
\\ \hsa do X=2*X
\So After applying the doubling $t$ times you will have $X=2^t$. So $T[N]$ is
one more than the
the maximal $t$ with $2^t < N$ which is one more than the maximal $t$ with $2^t\leq N-1$
which is the maximal $t$ with $t \leq lg(N-1)$ which is $1+\lfloor lg(N-1) \rfloor$.
(Almost full credit if you missed the $-1$.)
\item
FOR I=1 TO N
\\ \hsa X=1
\\ \hsa WHILE $X^2 \leq I$
\\ \hsb $X++$
\So For each $I$ you will apply the $X++$ step when $X\leq \sqrt{I}$ so
$\lfloor \sqrt{I} \rfloor$ times. Thus
\[ T(N) = \sum_{I=1}^N \lfloor \sqrt{I} \rfloor \]
Without the floor the splitting in half methods makes $T(N) = \Theta(N^{3/2})$.
The floors only effect each term by at most one and therefore the sum by at
most $N$ which is negligible compared to $\Theta(N^{3/2})$ so the answer is
$T(N)=\Theta(N^{3/2})$.
\een
\item (10) In hashing, what are collisions? Describe one
method (your choice!) for dealing with them.
\So Collisions are when there are two items $x,y$ which
have the same hash value, that is, $h(x)=h(y)$. There are
two methods: One is to have the hash table be a table of
linked lists and simply add $y$ to the linked list. The
other is to have a probe sequence, if $h(y)$ fails you have
backups $h_1(y),h_2(y),\ldots$ that you try.
\item (20) Let $A$ is a max-heap with heapsize $N$. Describe a
program called here
{\tt BIGGULP(A,i,key)} that replaces $A[i]$ by
a value $key$ which is bigger than $A[i]$ and then
restores the heap property. How long does {\tt BIGGULP}
take? How long does {\tt BIGGULP} take in the special
case when $i=1$?
\So Set $y=i$ and reset $A[y]\leftarrow key$. Now while
$y\neq 1$ you check whether $A[parent[y]] < A[y]$. If
it is you interchange $A[parent[y]]$ and $A[y]$ and
reset $y$ to $parent[y]$. Else you stop.
When $i=1$ you stop immediately so it takes $1$ (or $O(1)$) steps.
(Its bad form to say it takes zero steps because, after all,
you have to check that $i=1$.)
\item (15) You want to sort five elements $a,b,c,d,e$ using
seven paired comparisons. {\em Assume} that your question is
``Is $a** 2^5$ so we
cannot succeed by the Information Theoretic Lower Bound.
\item (15) There is an algorithm {\tt RABBIT(A,B)} that multiplies
two $n\times n$ matrices $A,B$ by performing seven multiplications
of $(n/2)\times (n/2)$ matrices and then performing $O(n^2)$
further operations. Create a recursive equation for the time
$T(n)$ that {\tt RABBIT(A,B)} takes and use the Master Theorem
to give $T(n)$ asymptotically.
\So This is Strassen's Algorithm. The recursion is
\[ T(n) = 7T(n/2) + O(n^2) \]
which is the low overhead case of the Master Theorem so that
$T(n)=\Theta(n^{\alpha})$ where $\alpha = \log_27$.
\item (15) Let $A[1\cdots N]$ be an array with all entries integers
between $0$ and $N$.
How long would {\tt RADIX-SORT} take to
sort $A$ assuming that we use base $2$ (that is, binary)?
(Assume the enries $A[I]$ are already given as binary
strings in the input.)
You
{\em must} give an argument for your answer.
\So Each counting sort would take time $O(N)$. But there are
$\lg N$ binary digits so the total time is $O(N\lg N)$.
\item (5) State the binary-search-tree property. (That is, the condition that
the keys are required to fulfill.)
\So For any node $x$ and any node $y$ in the left subtree of $x$, the value
at $y$ is $\leq$ the value at $x$.
For any node $x$ and any node $y$ in the right subtree of $x$, the value
at $y$ is $\geq$ the value at $x$.
\een
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