1826. faulty sensor

An experiment is being conducted in a lab. To ensure accuracy, there are two sensors collecting data simultaneously. You are given two arrays sensor1 and sensor2, where sensor1[i] and sensor2[i] are the ith data points collected by the two sensors.

However, this type of sensor has a chance of being defective, which causes exactly one data point to be dropped. After the data is dropped, all the data points to the right of the dropped data are shifted one place to the left, and the last data point is replaced with some random value. It is guaranteed that this random value will not be equal to the dropped value.

• For example, if the correct data is [1,2,**3**,4,5] and 3 is dropped, the sensor could return [1,2,4,5,**7**] (the last position can be any value, not just 7).

We know that there is a defect in at most one of the sensors. Return the sensor number (1 or 2) with the defect. If there is no defect in either sensor or if it is impossible to determine the defective sensor, return -1.

solution

Two pointers, but just a lot of edge cases.

def badSensor(self, sensor1: List[int], sensor2: List[int]) -> int:
	n = len(sensor1)
	if n < 2:
		return -1
	  
	p1 = p2 = 0
	while p1 < n and p2 < n:
		if sensor1[p1] == sensor2[p2]:
			p1 += 1
			p2 += 1
		else:
			# if we've reached last two values, we can't determine anything
			if p1+1 == n:
				return -1
			# can't determine yet
			if sensor1[p1+1] == sensor2[p2] and sensor1[p1] == sensor2[p2+1]:
				p1 += 1
				p2 += 1
			# needed to slide p1 up to match, so 2 was faulty
			elif sensor1[p1+1] == sensor2[p2]:
				return 2
			elif sensor1[p1] == sensor2[p2+1]:
				return 1
	return -1