19. As shown in the figure, fill in the circles with the numbers 1 to 6, so that the number in each circle is equal to the difference (larger minus smaller) of the numbers in the two adjacent circles below it, of course, the numbers in the three circles at the bottom do not have to follow this rule (these three circles do not have adjacent circles below them). Therefore, the number in the top circle is $\qquad$ (if there are multiple answers, write them all out)
【Answer】1 or 2 or 3
【Analysis】Key Point: Number Decomposition
First, consider that the top circle absolutely cannot contain the numbers $6,5,4$. We just need to test whether 1,2,3 are possible.