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I have to find which of the following values can be the degrees of an undirected graph with 6 vertices:

a) 3 2 2 2 3 3

b) 4 2 2 2 3 2

c) 5 2 2 2 0 3

d) 5 2 2 2 1 2

I only method I found is to try to draw the graph on a sheet of paper and then check if it is possible.

I just need a hint to start this problem, if possible, in other way than drawing each graph.