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\begin{center} \Large
{\scshape 6.851 Advanced Data Structures (Spring'07)} \\[1ex]
{Prof.~Erik Demaine \quad\quad TA: Oren Weimann} \\[2ex]
\framebox{Problem 2} \quad {\em Due: Monday, Feb. 26}
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Be sure to read the instructions on the assignments section of the
class web page.
\paragraph{\boldmath Wilber~1 is not good enough.}
Suppose we have a BST (such as Tango trees) whose running time
on an access sequence is upper bounded by some function $T(m,n,k)$
of the number $m$ of accesses, the number $n$ of keys, and
the number $k$ of interleaves incurred by each access.
(Assume for simplicity that every access incurs the same number of
interleaves.)
Determine the number $S(m,n,k)$ of different access sequences with $m$ accesses,
$n$ keys, and where each access incurs exactly $k$ interleaves.
%% Answer: {\lg n \choose k}^m ... actually (sum_{i=k}^{\lg n} {i \choose k})^m
Conclude that $T(m,n,k) = \Omega(\lg S(m,n,k))$,
and compute the resulting lower bound on~$T$.
%% Answer: m lg ((lg n) / k)
\paragraph{\boldmath Link-cut trees with LCA.}
Recall that the \emph{least common ancestor} (LCA) of two nodes $u$ and $v$
in a rooted tree $T$ is the deepest node in $T$ that is an ancestor of
both $u$ and~$v$. Describe how to modify link-cut trees in order to support
efficient LCA$(u,v)$ queries: given two nodes $u$ and~$v$, find their LCA.
You should support LCA queries in $O(\lg n)$ amortized time per operation,
while preserving the $O(\lg n)$ amortized cost per link, cut, and findroot.
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