Handshake Edge Count

Problem

Recover the number of undirected edges from a degree sequence using the handshake lemma.

Each undirected edge contributes 2 to the degree sum, so return total // 2. If the sum is odd the sequence cannot come from any graph, so return -1 instead.

Hint

Sum the degrees, reject odd sums, then divide by two.

Test cases

case 1

in: [[2,2,2]]

expected: 3

case 2

in: [[1,1]]

expected: 1

case 3

in: [[]]

expected: 0

case 4

in: [[3]]

expected: -1

case 5

in: [[0,0,0]]

expected: 0

Starter codePython

def edge_count_from_degrees(degrees):
    # Your code here
    pass

5 test cases · run your code against them in the editor