Drawing a diagram with tikz

Question

I am trying to draw a diagram like this:

So far what I got is this:

No idea where that little box in the top left corner is coming from...
This is my code:
documentclass[a4paper,10pt]{article}
usepackage{tikz}
usetikzlibrary{matrix,shapes,arrows,positioning,chains}

begin{document}
    tikzset{
    block/.style={
        rectangle,
        draw,
        text width=10em,
        text centered,
        rounded corners
    },
}

begin{tikzpicture}
matrix (m)[matrix of nodes, column  sep=1cm,row  sep=8mm, align=center, nodes={rectangle,draw, anchor=center} ]{
    
    & |[block]| {1}
    &  
     & & 
    |[block]| {1}
    & |[block]| {2}               
    & |[block]| {3}    
};
draw [>=latex,-] (m-1-2) |- (m-3-1);
end{tikzpicture}

end{document}

When I try to draw from the top box to the matrix element in the middle I get the error no shape named m-2-2 is known.
What should I do? Thanks

Zarko · Accepted Answer

You have empty lines in matrix code, it is the cause of undesired square in your image (in matrix you had not have empty lines)
for naming of all cells you need to add option nodes in empty cells, to the matrix styles options
that in empty cells are not drawn small squares, you had to change nodes style to coordinate:

documentclass[a4paper,10pt]{article}
usepackage{tikz}
usetikzlibrary{arrows,
                chains, 
                matrix,
                positioning}

begin{document}
    tikzset{
    block/.style={
        rectangle,
        draw,
        text width=10em,
        text centered,
        rounded corners
    },
}

begin{tikzpicture}
matrix (m) [matrix of nodes, 
             nodes in empty cells,
             column  sep=1cm,
             row  sep=8mm, 
             align=center, 
             nodes={coordinate}
             ]
{
                & |[block]| {1} &               
                &               &               
|[block]| {1}   & |[block]| {2} & |[block]| {3} 
};
draw       (m-1-2) -- (m-2-2) -| (m-3-1)
                              (m-2-2) -| (m-3-2)
                              (m-2-2) -| (m-3-3);
% or 
%   draw   (m-1-2) -- (m-3-2)
%           (m-3-1) |- (m-2-2) -| (m-3-3);
    end{tikzpicture}

end{document}

(In nodes in your images is higher, but in MWE you not define their heights. I follow node style definition in your MWE.)
However, your diagram is tree, which can be simpler drawn as tree, for example with forest package:
%documentclass[a4paper,10pt]{article}
documentclass[margin=3.141592]{standalone}
usepackage[edges]{forest}

begin{document}
    begin{forest}
    for tree={              
% style of nodes
      draw, semithick, rounded corners,
       text width = 4em, text badly centered,% changed 
   minimum height = 4ex,                     % added
% style of tree 
             edge = {draw=blue},             % changed
           anchor = north,
             grow = south,
    forked edge,            
            s sep =  4mm,   % horizontal ("sibling") distance between nodes
            l sep = 12mm,   % vertical ("level") distance between n odes
         fork sep =  6mm,   % distance from parent to branching point
               }
[1
    [1]
    [2]
    [3]
]
    end{forest}
end{document}

which drawn the following tree:

js bibra · Answer

documentclass{article}
usepackage{tikz} 
usetikzlibrary{arrows.meta, positioning, shapes} 
begin{document}
begin{tikzpicture}[
    node distance = 1.5cm and 1cm,
    arr/.style = {draw=blue!50, ultra thick},
    base/.style = {draw, font=small,
        minimum height=3em, text width=5em, align=center},
    block/.style = {base, fill=#1},
    block/.default = white,
    every edge/.append style = {arr}
    ]
    node [block] (A) {};
    node [block, below =of A] (B) {};
    node [block, right = of B] (C) {};
    node [block, left = of B] (D) {};
    coordinate[below=1cm of A] (aux);
    path[arr] (A) to (B);
    path[arr](aux) -| (C);
    path[arr](aux) -| (D);
end{tikzpicture}
end{document}

Drawing a diagram with tikz

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