Longest Alternating Subarray.py

You are given a 0-indexed integer array nums. A subarray s of length m is called alternating if:

m is greater than 1.
s1 = s0 + 1.
The 0-indexed subarray s looks like [s0, s1, s0, s1,...,s(m-1) % 2]. In other words, s1 - s0 = 1, s2 - s1 = -1, s3 - s2 = 1, s4 - s3 = -1, and so on up to s[m - 1] - s[m - 2] = (-1)m.
Return the maximum length of all alternating subarrays present in nums or -1 if no such subarray exists.

A subarray is a contiguous non-empty sequence of elements within an array.


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class Solution:
    def alternatingSubarray(self, nums: List[int]) -> int:

        n = len(nums)

        res = dp = -1

        for i in range(1,n):
            if dp>0 and nums[i] == nums[i-2]:
                dp+=1
            else:
                dp = 2 if nums[i] == nums[i-1]+1 else -1
            
            res = max(res, dp)
        return res