absolute value is only when a value there is no difference with sign bit.

where u got that concept "cannot take value greater than no. of vertex"

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A tree with n vertices is called graceful, if its vertices can be labelled with integers 1, , ..., n such that the absolute value of the difference of the labels of adjacent vertices are all different. Which of the following trees are graceful?

- a and b
- b and c
- a and c
- a, b,and c

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