Longest Strictly Increasing or Strictly Decreasing Subarray

main.md

Question

Approach

My first approach was to traverse forward and backward to check the longest monotonic subarray but this approach will take a long time as I will have to traverse twice. Instead, I'll used two pointer.

I used two variables, inc_len and dec_len, to track the length of the current increasing and decreasing subarrays, respectively. As I traverse the array, I compare each element with the previous one. If the current element is greater, I increment inc_len and reset dec_len to 1. If it is smaller, I increment dec_len and reset inc_len to 1. If they are equal, I reset both counters to 1.

Implementation

class Solution:
    def longestMonotonicSubarray(self, nums: List[int]) -> int:
        if not nums:
            return None

        inc_len, dec_len = 1, 1
        max_len = 1

        for i in range(1, len(nums)):
            if nums[i] > nums[i - 1]:
                inc_len += 1
                dec_len = 1  
            elif nums[i] < nums[i - 1]: 
                dec_len += 1
                inc_len = 1 
            else: 
                inc_len, dec_len = 1, 1

            max_len = max(max_len, inc_len, dec_len)
        
        return max_len

Complexities

• Time: O(n)
• Space: O(1)