\documentclass{minimal}

\usepackage{tikz}
\usetikzlibrary{matrix,calc}
\tikzset{
    every path/.style={dashed},
    every picture/.style={baseline},
    mymatrix/.style={
        matrix of math nodes,
        left delimiter=(,
        right delimiter=),
        every cell/.style={anchor=base east},
    },
}

\begin{document}
\begin{center}

\begin{tikzpicture}
    \matrix (m) [mymatrix] {
        0 & 1 & 2 & -1 \\
        0 & 0 & 0 &  1 \\
        0 & 0 & 0 &  0 \\
    };

    \draw (m-1-1.north east) |- ($(m-2-3.north)!.5!(m-2-4.north)$) |- ($(m-2-4.east)!.5!(m-3-4.east)$);
\end{tikzpicture},
\begin{tikzpicture}
    \matrix (m) [mymatrix] {
        1 & 2 & 1 & -1 & 2 \\
        0 & 0 & 1 &  0 & 2 \\
        0 & 0 & 0 &  2 & 3 \\
    };

    \draw (m-1-1.south west) -| ($(m-2-2.south)!.5!(m-2-3.south)$) -| ($(m-3-3.south)!.5!(m-3-4.south)$);
\end{tikzpicture},
\begin{tikzpicture}
    \matrix (m) [mymatrix] {
        1 & 0 & -1 \\
        0 & 2 &  1 \\
        0 & 0 &  3 \\
    };

    \draw ($(m-1-1.west)!.5!(m-2-1.west)$) -| ($(m-2-1.south)!.5!(m-2-2.south)$) -| ($(m-3-2.south)!.5!(m-3-3.south)$);
\end{tikzpicture}

\end{center}
\end{document}