  In this paper we address the constrained longest common subsequence problem.
Given two sequences $X$, $Y$ and a constrained sequence $P$, a sequence $Z$ is
a constrained longest common subsequence for $X$ and $Y$ with respect to $P$ if
$Z$ is the longest subsequence of $X$ and $Y$ such that $P$ is a subsequence of
$Z$. Recently, Tsai \cite{Tsai} proposed an $O(n^2 \cdot m^2 \cdot r)$ time
algorithm to solve this problem using dynamic programming technique, where $n$,
$m$ and $r$ are the lengths of $X$, $Y$ and $P$, respectively. In this paper,
we present a simple algorithm to solve the constrained longest common
subsequence problem in $O(n \cdot m \cdot r)$ time and show that the
constrained longest common subsequence problem is equivalent to a special case
of the constrained multiple sequence alignment problem which can also be
solved.
