Is Faulty Sensor easy or hard?

Faulty Sensor is classified as an Easy problem on LeetCode. The challenge comes from correctly identifying the first mismatch and reasoning about how a dropped value shifts the remainder of the array. Once that observation is made, the implementation is straightforward.

Faulty Sensor Python/Java solution

Both Python and Java implementations follow the same pattern: scan until the first mismatch, then verify alignment by skipping one element in either array. The logic uses simple loops and index comparisons, keeping the runtime O(n) and memory usage O(1).

How to solve Faulty Sensor in O(n)?

Iterate through both arrays until you find the first index where the readings differ. From that point, check whether skipping one value in sensor1 aligns the rest with sensor2 or skipping one value in sensor2 aligns the rest with sensor1. If exactly one alignment works, return that sensor index; otherwise return -1.

What is the best approach for Faulty Sensor?

The optimal approach is a single-pass traversal that finds the first mismatch between the two arrays and then checks which array becomes aligned when one element is skipped. This method runs in O(n) time and O(1) space because it only scans the remaining portion once. Interviewers typically expect this linear check instead of repeated comparisons.

Is Faulty Sensor asked at Google/Amazon/Meta?

Faulty Sensor represents a typical array comparison and edge‑case reasoning problem often used in technical screens. Variants of shifted array detection or mismatch detection appear in interviews at companies like Amazon and Google because they test careful traversal logic and boundary handling.

What data structure is used in Faulty Sensor?

The problem primarily uses arrays and simple index traversal. A two-pointer style comparison is applied after the first mismatch to check which sequence shifted by one element. No additional data structures such as hash maps or stacks are required.

What is the time complexity of Faulty Sensor?

The optimal solution runs in O(n) time where n is the number of readings in the arrays. You scan the arrays until the first mismatch and then compare the remaining elements once more. The algorithm uses O(1) additional space because it only stores a few indices.

Description Hints 3 Solution Video 6:44

1826. Faulty Sensor

Easy50.2% acceptance1 companyPremium elsewhere, free here

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 i^th 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.

Example 1:

Input: sensor1 = [2,3,4,5], sensor2 = [2,1,3,4]
Output: 1
Explanation: Sensor 2 has the correct values.
The second data point from sensor 2 is dropped, and the last value of sensor 1 is replaced by a 5.

Example 2:

Input: sensor1 = [2,2,2,2,2], sensor2 = [2,2,2,2,5]
Output: -1
Explanation: It is impossible to determine which sensor has a defect.
Dropping the last value for either sensor could produce the output for the other sensor.

Example 3:

Input: sensor1 = [2,3,2,2,3,2], sensor2 = [2,3,2,3,2,7]
Output: 2
Explanation: Sensor 1 has the correct values.
The fourth data point from sensor 1 is dropped, and the last value of sensor 1 is replaced by a 7.

Constraints:

sensor1.length == sensor2.length
1 <= sensor1.length <= 100
1 <= sensor1[i], sensor2[i] <= 100

Asked at 1 company

Topics

Array Two Pointers

Hints open one at a time

Hint 1

Check for a common prefix of the two arrays.

Hint 2

After this common prefix, there should be one array similar to the other but shifted by one.

Hint 3

If both arrays can be shifted, return -1.

6:44

Still stuck?Read the written solution, or watch a walkthrough.

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1class Solution:
  def solve(self):
      pass


Starter codePython 3Spaces: 4

Examples

Case 1Case 2Case 3

Input

[2,3,4,5]
[2,1,3,4]

Input

[2,2,2,2,2]
[2,2,2,2,5]

Input

[2,3,2,2,3,2]
[2,3,2,3,2,7]

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