How to create a small cell with borders in the top corner of a cell in the table?

Question

How to get the node at the top corner of the cell in the table with borders and does not mess with the alignment of the empty cell in a table?

\documentclass[12pt]{standalone}
\usepackage{tikz}
\usepackage{array}
\begin{document}
\begin{tabular}{|c|c|c|c|c}\cline{1-4} 
20 \raisebox{0.2cm}{\small 4} & \raisebox{0.2cm}{\small 6} & \raisebox{0.2cm}{\small 8} & \raisebox{0.2cm}{\small 8} & 40\\ 
\cline{1-4} 
& & & & 60\\ \cline{1-4} 
& & & & 50 \\ \cline{1-4} 
\multicolumn{1}{c}{20} & \multicolumn{1}{c}{30} & \multicolumn{1}{c}{50} & \multicolumn{1}{c}{50} & 
150\\ 
\end{tabular}

Answer 1

With TikZ this is very easy. This answer defines a style inlay that can be used for those. You can add to the respective cell

|[inlay=<content>]|

Code:

\documentclass[tikz,border=3mm]{standalone}
\usepackage{amsmath}
\usetikzlibrary{matrix}
\begin{document}
\begin{tikzpicture}[mmat/.style={matrix of math nodes,nodes in empty cells,
        row sep=-\pgflinewidth,column sep=-\pgflinewidth,
        nodes={minimum width=5.5em,minimum height=3.5em,
            draw,anchor=center,
            text depth=0.25ex,text height=0.8em}},
        inlay/.style={label={[draw,thin,anchor=north east,minimum width=0em,
        minimum height=0em,inner sep=1.4pt]north east:#1}}]
 \matrix[mmat] (mat){
 &  P & Q & R & S & \textit{Supply} \\
 A & |[inlay=180]| 12& |[inlay=150]| 10 & |[inlay=170]| 12 & 13 & 500 \\
 };         
 \draw (mat-1-1.north west) -- (mat-1-1.south east);
 \draw[stealth-] ([yshift=2ex,xshift=-0.5ex]mat-1-1.east) 
    -- ++ (-2ex,0) node[left,font=\itshape]{To};
 \draw[stealth-] ([xshift=3ex,yshift=0.5ex]mat-1-1.south west) 
    -- ++ (0,2ex) node[above,font=\itshape]{From};
\end{tikzpicture}
\end{document}

Answer 2

Here is a solution with {NiceArray} of nicematrix. That environment is similar to the classical environment {nicearray} (of array) but creates PGF/Tikz nodes under the cells, rows and columns. I use these nodes to add the small corners after the construction of the array.

\documentclass{article}
\usepackage{nicematrix,tikz}
\ExplSyntaxOn
\makeatletter
\cs_new_protected:Nn __rakatex_draw_angle:nnn 
  { 
    \begin{tikzpicture}
    \draw ([xshift=-5.5mm]#1-|#2) |- ([yshift=-4mm]#1-|#2) ; 
    \node at ([xshift=-2.75mm,yshift=-2mm]#1-|#2) { \small #3 } ;
    \end{tikzpicture}
  }
\NewDocumentCommand{\Cell}{mm}
  {
    #1
    \tl_gput_right:Nx \g_nicematrix_code_after_tl
      { 
        __rakatex_draw_angle:nnn
          { \int_use:N \c@iRow } 
          { \int_eval:n { \c@jCol + 1 } } 
          { #2 } 
      }
  }
\makeatother
\ExplSyntaxOff
\begin{document}
\renewcommand{\arraystretch}{2}
$\begin{NiceArray}{c*{4}{wc{1.5cm}}c}[hvlines]
\diagbox{\downarrow\emph{From}}{\emph{To}\to}& P & Q & R & S & \text{\emph{Supply}} \
A & \Cell{12}{180} & \Cell{10}{150} & \Cell{12}{170} & 13 & 500 \
B & 7 & 11 & \Cell{8}{180} & \Cell{14}{120} & 300 \
C & 6 & 16 & 11 & \Cell{7}{200} & 200 \
\text{Demand} & 180 & 150 & 350 & 320 & 1000
\end{NiceArray}$
\end{document}