The second fastest train

The list speeds holds the top speed of each train. Return the second largest different speed. Equal values count as one speed. If there are fewer than two different speeds, return None.

second_largest([110, 130, 90, 120])   # 120
second_largest([130, 110, 130])       # 110: 130 counts once
second_largest([100, 100])            # None
second_largest([75])                  # None

Solve it in one pass, as in lesson 2. Keep two names, the best and the second best seen so far. Write down the invariant they keep, then check that every step keeps it.

Limits

• Up to 1,000,000 speeds. The list has at least one speed.
• Speeds can be negative. Do not sort or change speeds.