A lap timer records the cumulative distance a robot has driven in readings. The readings should never decrease. Return the index of the first reading that is smaller than the reading before it, or -1 if the readings never drop.
Examples
Input: readings = [1, 2, 3, 2, 5]
Output: 3
Explanation: readings[3] = 2 is smaller than readings[2] = 3.
Input: readings = [1, 1, 2]
Output: -1
Explanation: Staying equal is not a drop.
Constraints
0 <= len(readings) <= 10**5- Target: O(n) time; stop as early as possible.
Goals
- Return as soon as a condition is met
- Return a sentinel when the condition never occurs