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\begin{center} \Large
{\scshape 6.851 Advanced Data Structures (Spring'12)} \\[1ex]
{Prof.~Erik Demaine \qquad TAs: Tom Morgan, Justin Zhang} \\[2ex]
\framebox{Problem 9} \quad {\em Due: Thursday, Apr.\ 26}
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Be sure to read the instructions on the assignments section of the
class web page.  Remember to keep your solutions to one page!

\paragraph{Substring matching.}
Given two strings $S_1$ and $S_2$, and a positive integer $k$, find the number of substrings of $S_1$ of length at least $k$ that occur in $S_2$.  Develop and analyze an algorithm to solve this problem in $O(|S_1| + |S_2| + \mathrm{sort}(\Sigma))$ time.

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