Is Monotonic

Unit 2 Session 1 (Click for link to problem statements)

U-nderstand

Understand what the interviewer is asking for by using test cases and questions about the problem.

• Do I need to return anything different to distinguish between an increasing and decreasing list?
  • No. In both cases, your function should return True.
• What if the list is empty?
  • In this case, return True.

P-lan

Plan the solution with appropriate visualizations and pseudocode.

General Idea: Loop through each element in the list, and keep track of whether it consistently increases or descreases.

1) By default, assume the list is both increasing AND decreasing
2) Loop through each index in the list, starting at index 1
  a) Compare the current value to the previous value
      i) If previous < current, decreasing must be False
     ii) If previous > current, increasing must be False
3) If either increasing or decreasing is still True, return True
  

⚠️ Common Mistakes

• Since you need to check the previous/next element (depending on how you write your solution), make sure your loop doesn't go out of bounds!

I-mplement

def is_monotonic(nums):
    increasing = decreasing = True

    for i in range(1, len(nums)):
        if nums[i] > nums[i-1]:
            decreasing = False
        elif nums[i] < nums[i-1]:
            increasing = False

    return increasing or decreasing

*Note: You could potentially give this function a better best-case & average-case efficiency by exiting the loop early when both decreasing and increasing are false.

However, this won't effect the worst-case efficiency (Big-O) of the solution.*