Number of strokes = odd number divided by 2
Odd numbers are 0 and 2 are strokes.
When the odd number is even > 2, the number of strokes is divided by 2.
When the odd number is greater than 2 (3, 5, 7, 9 ...), divide by 2, and the result quotient+1 gets the number of strokes (because the remainder is even smaller! )。
For example, the number of singularities in a graph is 0 or 2 (singularities, points leading to odd lines).
According to the above two conditions, it can be judged that ① is not a connected graph, ④ has four singularities, and ⑤ has six singularities, all of which cannot be drawn with one stroke, while ② and ③ have 0 and 2 singularities, respectively, which can be drawn with one stroke.