LeetCode 2210: Count Hills and Valleys in an Array ·

Table of Contents

Meta Data
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Difficulty: easy First Attempt: 2025-07-27

Submissions: Accepted (first try)

Approach
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Equal neighbors belong to the same hill or valley, so look only at directions: each adjacent pair goes up, down or flat (Array Template 1). Skip flat pairs and remember the last non-flat direction; up turning down is a hill, down turning up a valley.

My Solution
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class Solution:
    def countHillValley(self, nums: List[int]) -> int:
        res, prev = 0, 0                       # prev：上一個不平的方向，1 上、-1 下
        for a, b in zip(nums, nums[1:]):
            d = (b > a) - (b < a)
            if d == 0:
                continue
            if prev and d != prev:
                res += 1
            prev = d
        return res

I originally stored the directions as "U", "D", "T" in a list and scanned it again; same logic.

Complexity
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Time O(n), space O(n) (the nums[1:] copy).

Mistakes to Avoid
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Equal neighbors are not new hills or valleys; skip them.

Interview Explanation
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I look at the direction of each adjacent pair, skip equal ones, and remember the last non-flat direction; each change of direction adds one hill or valley. Time O(n).