6. Arrange the positive integers in the following array:
\begin{tabular}{lllll}
1 & 2 & 5 & 10 & $\ldots$ \\
4 & 3 & 6 & 11 & $\ldots$ \\
9 & 8 & 7 & 12 & $\ldots$ \\
16 & 15 & 14 & 13 & $\ldots$ \\
$\ldots$ & $\ldots$ & $\ldots$ & $\ldots$ & $\ldots$
\end{tabular}

What is the number in the 21st row and 21st column?
【Analysis】The first column number of each row is a perfect square, so the number in the first column of the 21st row is $21 \times 21=441$. Therefore, the number in the 21st column of the 21st row is $441-(21-1)=421$.