eth-summaries/semester1/algorithms-and-datastructures/parts/graphs/shortest-path.tex

\newpage
\subsection{Shortest path}
\subsubsection{Dijkstra's Algorithm}
\begin{definition}[]{Dijkstra's Algorithm}
    \textbf{Dijkstra's Algorithm} is a graph search algorithm that finds the shortest paths from a source vertex to all other vertices in a graph with non-negative edge weights.
\end{definition}

\begin{algorithm}
    \caption{Dijkstra's Algorithm}
    \begin{algorithmic}[1]
        \Procedure{Dijkstra}{$G = (V, E), s$} \Comment{$s$ is the source vertex}
            \State Initialize distances: $d[v] \gets \infty \; \forall v \in V, d[s] \gets 0$
            \State Initialize priority queue $Q$ with all vertices, priority set to $\infty$
            \While{$Q$ is not empty}
                \State $u \gets \textbf{Extract-Min}(Q)$
                \For{each neighbor $v$ of $u$}
                    \If{$d[u] + w(u, v) < d[v]$} \Comment{If weight through current vertex is lower, update}
                        \State $d[v] \gets d[u] + w(u, v)$
                        \State Update $Q$ with new distance of $v$
                    \EndIf
                \EndFor
            \EndWhile
            \State \textbf{Return} distances $d$
        \EndProcedure
    \end{algorithmic}
\end{algorithm}

\begin{properties}[]{Characteristics of Dijkstra's Algorithm}
    \begin{itemize}
        \item \textbf{Time Complexity:}
              \begin{itemize}
                  \item \tco{|V|^2} for a simple implementation.
                  \item \tco{(|V| + |E|) \log |V|} using a priority queue.
              \end{itemize}
        \item Only works with non-negative edge weights.
        \item Greedy algorithm that processes vertices in increasing order of distance.
        \item Common applications include navigation systems and network routing.
    \end{itemize}
\end{properties}

\begin{usage}[]{Dijkstra's Algorithm}
    Dijkstra's algorithm finds the shortest path from a source vertex to all other vertices in a weighted graph (non-negative weights).

    \begin{enumerate}
        \item \textbf{Initialize:}
              \begin{itemize}
                  \item Set the distance to the source vertex as 0 and to all other vertices as infinity.
                  \item Mark all vertices as unvisited.
              \end{itemize}

        \item \textbf{Start from Source:}
              \begin{itemize}
                  \item Select the unvisited vertex with the smallest tentative distance.
              \end{itemize}

        \item \textbf{Update Distances:}
              \begin{itemize}
                  \item For each unvisited neighbor of the current vertex:
                        \begin{itemize}
                            \item Calculate the tentative distance through the current vertex.
                            \item If the calculated distance is smaller than the current distance, update it.
                        \end{itemize}
              \end{itemize}

        \item \textbf{Mark as Visited:}
              \begin{itemize}
                  \item Mark the current vertex as visited. Once visited, it will not be revisited.
              \end{itemize}

        \item \textbf{Repeat:}
              \begin{itemize}
                  \item Repeat steps 2-4 until all vertices are visited or the shortest path to all vertices is determined.
              \end{itemize}

        \item \textbf{End:}
              \begin{itemize}
                  \item The algorithm completes when all vertices are visited or when the shortest paths to all reachable vertices are found.
              \end{itemize}
    \end{enumerate}
\end{usage}




\newpage
\subsubsection{Bellman-Ford Algorithm}
\begin{definition}[]{Bellman-Ford Algorithm}
    The \textbf{Bellman-Ford Algorithm} computes shortest paths from a source vertex to all other vertices, allowing for graphs with negative edge weights.
\end{definition}

\begin{algorithm}
    \caption{Bellman-Ford Algorithm}
    \begin{algorithmic}[1]
        \Procedure{Bellman-Ford}{$G = (V, E), s$} \Comment{$s$ is the source vertex}
            \State Initialize distances: $d[v] \gets \infty \; \forall v \in V, d[s] \gets 0$
            \For{$i \gets 1$ to $|V| - 1$} \Comment{Relax all edges $|V| - 1$ times}
                \For{each edge $(u, v, w(u, v)) \in E$}
                    \If{$d[u] + w(u, v) < d[v]$}
                        \State $d[v] \gets d[u] + w(u, v)$
                    \EndIf
                \EndFor
            \EndFor
            \For{each edge $(u, v, w(u, v)) \in E$} \Comment{Check for negative-weight cycles}
                \If{$d[u] + w(u, v) < d[v]$}
                    \State \textbf{Report Negative Cycle}
                \EndIf
            \EndFor
            \State \Return distances $d$
        \EndProcedure
    \end{algorithmic}
\end{algorithm}

\begin{properties}[]{Characteristics of Bellman-Ford Algorithm}
    \begin{itemize}
        \item \textbf{Time Complexity:} \tco{|V| \cdot |E|}.
        \item Can handle graphs with negative edge weights but not graphs with negative weight cycles.
        \item Used for:
              \begin{itemize}
                  \item Detecting negative weight cycles.
                  \item Computing shortest paths in graphs where Dijkstra’s algorithm is not applicable.
              \end{itemize}
    \end{itemize}
\end{properties}

\begin{usage}[]{Bellman-Ford Algorithm}
    The Bellman-Ford algorithm finds the shortest path from a source vertex to all other vertices in a weighted graph (handles negative weights).

    \begin{enumerate}
        \item \textbf{Initialize:}
              \begin{itemize}
                  \item Set the distance to the source vertex as 0 and to all other vertices as infinity.
              \end{itemize}

        \item \textbf{Relax Edges:}
              \begin{itemize}
                  \item Repeat for \(V-1\) iterations (where \(V\) is the number of vertices):
                        \begin{itemize}
                            \item For each edge, update the distance to its destination vertex if the distance through the edge is smaller than the current distance.
                        \end{itemize}
              \end{itemize}

        \item \textbf{Check for Negative Cycles:}
              \begin{itemize}
                  \item Check all edges to see if a shorter path can still be found. If so, the graph contains a negative-weight cycle.
              \end{itemize}

        \item \textbf{End:}
              \begin{itemize}
                  \item If no negative-weight cycle is found, the algorithm outputs the shortest paths.
              \end{itemize}
    \end{enumerate}
\end{usage}



\begin{properties}[]{Comparison of Dijkstra and Bellman-Ford}
    \begin{center}
        \begin{tabular}{lcc}
            \toprule
            \textbf{Feature}         & \textbf{Dijkstra's Algorithm} & \textbf{Bellman-Ford Algorithm} \\
            \midrule
            Handles Negative Weights & No                            & Yes                             \\
            Detects Negative Cycles  & No                            & Yes                             \\
            Time Complexity          & \tco{(|V| + |E|) \log |V|}    & \tco{|V| \cdot |E|}             \\
            Algorithm Type           & Greedy                        & Dynamic Programming             \\
            \bottomrule
        \end{tabular}
    \end{center}
\end{properties}