2393. Count Strictly Increasing Subarrays

1. Description

You are given an array nums consisting of positive integers.
Return the number of subarrays of nums that are in strictly increasing order.
A subarray is a contiguous part of an array.

2. Example

Example 1

Input: nums = [1,3,5,4,4,6]
Output: 10
Explanation: The strictly increasing subarrays are the following:

• Subarrays of length 1: [1], [3], [5], [4], [4], [6].
• Subarrays of length 2: [1,3], [3,5], [4,6].
• Subarrays of length 3: [1,3,5].

The total number of subarrays is 6 + 3 + 1 = 10.

Example 2

Input: nums = [1,2,3,4,5]
Output: 15
Explanation: Every subarray is strictly increasing. There are 15 possible subarrays that we can take.

3. Constraints

• 1 <= nums.length <= 10$^5$
  1 <= nums[i] <= 10$^6$

4. Solutions

Math

n = nums.size()
Time complexity: O(n)
Space complexity: O(1)

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class Solution {
public:
    long long countSubarrays(vector<int> &nums) {
        long long count = 1, length = 1;
        for (int i = 1, n = nums.size(); i < n; ++i) {
            if (nums[i] > nums[i - 1]) {
                ++length;
            } else {
                length = 1;
            }

            count += length;
        }

        return count;
    }
};