% \tikzset{
%     scale=1.0,
%     font=\normalsize,
%     level distance=1.2cm,
%     level 1/.style={sibling distance=2.4cm},
%     level 2/.style={sibling distance=0.95cm},
%     level 3/.style={sibling distance=0.95cm},
%     every node/.style={font=\normalsize},
%     leaf/.style={circle, draw=blue!50!black, fill=blue!10, minimum size=7mm, line width=0.7pt,
%     drop shadow={opacity=0.3, shadow xshift=1.5pt, shadow yshift=-1.5pt}},
%     internal/.style={circle, draw=gray!70!black, fill=gray!10, minimum size=8mm, line width=0.7pt,
%     drop shadow={opacity=0.3, shadow xshift=1.5pt, shadow yshift=-1.5pt}},
%     edge from parent/.style={draw, line width=0.8pt, gray!60}
% }

% \begin{tikzpicture}

\begin{tikzpicture}[
    scale=1.0,
    font=\normalsize,
    level distance=1.2cm,
    level 1/.style={sibling distance=2.4cm},
    level 2/.style={sibling distance=0.95cm},
    level 3/.style={sibling distance=0.95cm},
    every node/.style={font=\normalsize},
    leaf/.style={circle, draw=blue!50!black, fill=blue!10, minimum size=7mm, line width=0.7pt,
                 drop shadow={opacity=0.3, shadow xshift=1.5pt, shadow yshift=-1.5pt}},
    internal/.style={circle, draw=gray!70!black, fill=gray!10, minimum size=8mm, line width=0.7pt,
                     drop shadow={opacity=0.3, shadow xshift=1.5pt, shadow yshift=-1.5pt}},
    edge from parent/.style={draw, line width=0.8pt, gray!60}
]

% ==========================================
% PANEL A: By Node (4 groups: u1, u2, u3, u4)
% DFS order: u1 -> u2 -> u3 -> u4
% ==========================================
\begin{scope}[xshift=0cm]
    \node[font=\bfseries\Large] at (0, 1.2) {By Node};

    % Tree: u1 -> {1, u2, u3}, u2 -> {2, 3}, u3 -> {u4, 6, 7}, u4 -> {4, 5}
    \node[internal, fill=red!15, draw=red!60!black] (u1) at (0, 0) {$u_1$}
        child {node[leaf] (l1) {$1$}}
        child {node[internal, fill=green!15, draw=green!60!black] (u2) {$u_2$}
            child {node[leaf] (l2) {$2$}}
            child {node[leaf] (l3) {$3$}}
        }
        child {node[internal, fill=orange!15, draw=orange!60!black] (u3) {$u_3$}
            child {node[internal, fill=cyan!15, draw=cyan!60!black] (u4) {$u_4$}
                child {node[leaf] (l4) {$4$}}
                child {node[leaf] (l5) {$5$}}
            }
            child {node[leaf] (l6) {$6$}}
            child {node[leaf] (l7) {$7$}}
        };

    % V box - 6 coordinates: u1(2), u2(1), u3(2), u4(1)
    \begin{scope}[yshift=-5.0cm]
        % Braces above
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, red!60!black] 
            (-2.7, 0.15) -- (-0.9, 0.15) node[midway, above=4pt, font=\small, text=red!60!black] {$u_1$};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, green!60!black] 
            (-0.9, 0.15) -- (0.0, 0.15) node[midway, above=4pt, font=\small, text=green!60!black] {$u_2$};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, orange!60!black] 
            (0.0, 0.15) -- (1.8, 0.15) node[midway, above=4pt, font=\small, text=orange!60!black] {$u_3$};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, cyan!60!black] 
            (1.8, 0.15) -- (2.7, 0.15) node[midway, above=4pt, font=\small, text=cyan!60!black] {$u_4$};
        
        % Frame background
        \draw[fill=white, drop shadow={opacity=0.1}] (-2.7, 0) rectangle (2.7, -0.7);
        
        % Colored sections: 2 + 1 + 2 + 1 = 6 coords
        \fill[red!15] (-2.7, 0) rectangle (-0.9, -0.7);
        \fill[green!15] (-0.9, 0) rectangle (0.0, -0.7);
        \fill[orange!15] (0.0, 0) rectangle (1.8, -0.7);
        \fill[cyan!15] (1.8, 0) rectangle (2.7, -0.7);
        
        % Grid lines (6 cells)
        \draw[gray!30] (-1.8, 0) -- (-1.8, -0.7);
        \draw[gray!30] (-0.9, 0) -- (-0.9, -0.7);
        \draw[gray!30] (0.0, 0) -- (0.0, -0.7);
        \draw[gray!30] (0.9, 0) -- (0.9, -0.7);
        \draw[gray!30] (1.8, 0) -- (1.8, -0.7);
        
        % Frame
        \draw[thick] (-2.7, 0) rectangle (2.7, -0.7);
        
        % Label
        \node[font=\large, anchor=north] at (0, -0.9) {$V \in \mathbb{R}^{7 \times 6}$};
    \end{scope}
\end{scope}

% Separator 1
\draw[gray!20, line width=1.5pt] (5.0, 1.4) -- (5.0, -6.6);

% ==========================================
% PANEL B: By Depth (3 groups: depth 0, 1, 2)
% ==========================================
\begin{scope}[xshift=9.2cm]
    \node[font=\bfseries\Large] at (0, 1.2) {By Depth};

    % Tree with depth coloring: depth 0 = u1, depth 1 = u2/u3, depth 2 = u4
    \node[internal, fill=purple!15, draw=purple!60!black] (u1) at (0, 0) {$u_1$}
        child {node[leaf] (l1) {$1$}}
        child {node[internal, fill=teal!15, draw=teal!60!black] (u2) {$u_2$}
            child {node[leaf] (l2) {$2$}}
            child {node[leaf] (l3) {$3$}}
        }
        child {node[internal, fill=teal!15, draw=teal!60!black] (u3) {$u_3$}
            child {node[internal, fill=yellow!30, draw=yellow!70!black] (u4) {$u_4$}
                child {node[leaf] (l4) {$4$}}
                child {node[leaf] (l5) {$5$}}
            }
            child {node[leaf] (l6) {$6$}}
            child {node[leaf] (l7) {$7$}}
        };

    % V box - grouped by depth: depth0(2), depth1(1+2=3), depth2(1)
    \begin{scope}[yshift=-5.0cm]
        % Braces above
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, purple!60!black] 
            (-2.7, 0.15) -- (-0.9, 0.15) node[midway, above=4pt, font=\small, text=purple!60!black] {depth 0};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, teal!60!black] 
            (-0.9, 0.15) -- (1.8, 0.15) node[midway, above=4pt, font=\small, text=teal!60!black] {depth 1};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, yellow!70!black] 
            (1.8, 0.15) -- (2.7, 0.15) node[midway, above=4pt, font=\small, text=yellow!70!black] {depth 2};
        
        % Frame background
        \draw[fill=white, drop shadow={opacity=0.1}] (-2.7, 0) rectangle (2.7, -0.7);
        
        % Colored sections: depth0(2) + depth1(1+2=3) + depth2(1) = 6
        \fill[purple!15] (-2.7, 0) rectangle (-0.9, -0.7);
        \fill[teal!15] (-0.9, 0) rectangle (1.8, -0.7);
        \fill[yellow!30] (1.8, 0) rectangle (2.7, -0.7);
        
        % Grid lines
        \draw[gray!30] (-1.8, 0) -- (-1.8, -0.7);
        \draw[gray!30] (-0.9, 0) -- (-0.9, -0.7);
        \draw[gray!30] (0.0, 0) -- (0.0, -0.7);
        \draw[gray!30] (0.9, 0) -- (0.9, -0.7);
        \draw[gray!30] (1.8, 0) -- (1.8, -0.7);
        
        % Frame
        \draw[thick] (-2.7, 0) rectangle (2.7, -0.7);
        
        % Label
        \node[font=\large, anchor=north] at (0, -0.9) {$V \in \mathbb{R}^{7 \times 6}$};
    \end{scope}
\end{scope}

% Separator 2
\draw[gray!20, line width=1.5pt] (14.2, 1.4) -- (14.2, -6.6);

% ==========================================
% PANEL C: By Subtree (3 groups: u1, u2-subtree, u3-subtree)
% u3-subtree includes u3 AND u4
% ==========================================
\begin{scope}[xshift=18.4cm]
    \node[font=\bfseries\Large] at (0, 1.2) {By Subtree};

    % Tree: subtree coloring - u3 subtree includes u4 (leaves stay blue)
    \node[internal, fill=red!15, draw=red!60!black] (u1) at (0, 0) {$u_1$}
        child {node[leaf] (l1) {$1$}}
        child {node[internal, fill=green!15, draw=green!60!black] (u2) {$u_2$}
            child {node[leaf] (l2) {$2$}}
            child {node[leaf] (l3) {$3$}}
        }
        child {node[internal, fill=orange!15, draw=orange!60!black] (u3) {$u_3$}
            child {node[internal, fill=orange!20, draw=orange!60!black] (u4) {$u_4$}
                child {node[leaf] (l4) {$4$}}
                child {node[leaf] (l5) {$5$}}
            }
            child {node[leaf] (l6) {$6$}}
            child {node[leaf] (l7) {$7$}}
        };

    % V box - grouped by subtree: root(2), u2-subtree(1), u3-subtree(2+1=3) = 6
    \begin{scope}[yshift=-5.0cm]
        % Braces above
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, red!60!black] 
            (-2.7, 0.15) -- (-0.9, 0.15) node[midway, above=4pt, font=\small, text=red!60!black] {$u_1$};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, green!60!black] 
            (-0.9, 0.15) -- (0.0, 0.15) node[midway, above=4pt, font=\small, text=green!60!black] {$u_2$ subtree};
        \draw[decorate, decoration={brace, amplitude=4pt}, line width=0.6pt, orange!60!black] 
            (0.0, 0.15) -- (2.7, 0.15) node[midway, above=4pt, font=\small, text=orange!60!black] {$u_3$ subtree};
        
        % Frame background
        \draw[fill=white, drop shadow={opacity=0.1}] (-2.7, 0) rectangle (2.7, -0.7);
        
        % Colored sections: root(2) + u2-subtree(1) + u3-subtree(3) = 6
        \fill[red!15] (-2.7, 0) rectangle (-0.9, -0.7);
        \fill[green!15] (-0.9, 0) rectangle (0.0, -0.7);
        \fill[orange!15] (0.0, 0) rectangle (2.7, -0.7);
        
        % Grid lines
        \draw[gray!30] (-1.8, 0) -- (-1.8, -0.7);
        \draw[gray!30] (-0.9, 0) -- (-0.9, -0.7);
        \draw[gray!30] (0.0, 0) -- (0.0, -0.7);
        \draw[gray!30] (0.9, 0) -- (0.9, -0.7);
        \draw[gray!30] (1.8, 0) -- (1.8, -0.7);
        
        % Frame
        \draw[thick] (-2.7, 0) rectangle (2.7, -0.7);
        
        % Label
        \node[font=\large, anchor=north] at (0, -0.9) {$V \in \mathbb{R}^{7 \times 6}$};
    \end{scope}
\end{scope}

\end{tikzpicture}