LeetCode 2210. Count Hills and Valleys in an Array

原题链接在这里：https://leetcode.com/problems/count-hills-and-valleys-in-an-array/description/

题目：

You are given a 0-indexed integer array nums. An index i is part of a hill in nums if the closest non-equal neighbors of i are smaller than nums[i]. Similarly, an index i is part of a valley in nums if the closest non-equal neighbors of i are larger than nums[i]. Adjacent indices i and j are part of the same hill or valley if nums[i] == nums[j].

Note that for an index to be part of a hill or valley, it must have a non-equal neighbor on both the left and right of the index.

Return the number of hills and valleys in nums.

Example 1:

Input: nums = [2,4,1,1,6,5]
Output: 3
Explanation:
At index 0: There is no non-equal neighbor of 2 on the left, so index 0 is neither a hill nor a valley.
At index 1: The closest non-equal neighbors of 4 are 2 and 1. Since 4 > 2 and 4 > 1, index 1 is a hill. 
At index 2: The closest non-equal neighbors of 1 are 4 and 6. Since 1 < 4 and 1 < 6, index 2 is a valley.
At index 3: The closest non-equal neighbors of 1 are 4 and 6. Since 1 < 4 and 1 < 6, index 3 is a valley, but note that it is part of the same valley as index 2.
At index 4: The closest non-equal neighbors of 6 are 1 and 5. Since 6 > 1 and 6 > 5, index 4 is a hill.
At index 5: There is no non-equal neighbor of 5 on the right, so index 5 is neither a hill nor a valley. 
There are 3 hills and valleys so we return 3.

Example 2:

Input: nums = [6,6,5,5,4,1]
Output: 0
Explanation:
At index 0: There is no non-equal neighbor of 6 on the left, so index 0 is neither a hill nor a valley.
At index 1: There is no non-equal neighbor of 6 on the left, so index 1 is neither a hill nor a valley.
At index 2: The closest non-equal neighbors of 5 are 6 and 4. Since 5 < 6 and 5 > 4, index 2 is neither a hill nor a valley.
At index 3: The closest non-equal neighbors of 5 are 6 and 4. Since 5 < 6 and 5 > 4, index 3 is neither a hill nor a valley.
At index 4: The closest non-equal neighbors of 4 are 5 and 1. Since 4 < 5 and 4 > 1, index 4 is neither a hill nor a valley.
At index 5: There is no non-equal neighbor of 1 on the right, so index 5 is neither a hill nor a valley.
There are 0 hills and valleys so we return 0.

Constraints:

• 3 <= nums.length <= 100
• 1 <= nums[i] <= 100

题解：

Have a maker d to mark if it is previously increasing (d = 1) or decreasing (d = -1).
Check current element nums[i], if it is > nums[i - 1], need to check if we have a new hill, if d == -1, we have a new hill. And mark d as 1 now.

Vice versa.

Time Complexity: O(n). n = nums.length;

Space: O(1).

AC Java:

class Solution {
    public int countHillValley(int[] nums) {
        if(nums == null || nums.length < 3){
            return 0;
        }

        int n = nums.length;
        int d = 0;
        int res = 0;
        for(int i = 1; i < n; i++){
            if(nums[i] > nums[i - 1]){
                if(d < 0){
                    res++;
                }

                d = 1;
            }

            if(nums[i] < nums[i - 1]){
                if(d > 0){
                    res++;
                }

                d = -1;
            }
        }

        return res;
    }
}

类似Monotonic Array.