LeetCode Entry

2110. Number of Smooth Descent Periods of a Stock

15.12.2025 medium 2025 kotlin rust

Decreasing subarrays

2110. Number of Smooth Descent Periods of a Stock medium blog post substack youtube

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Problem TLDR

Decreasing subarrays #medium

Intuition

Just count them like and arithmetic progression.

Approach

• res += count++

Complexity

• Time complexity: \(O(n)\)

• Space complexity: \(O(1)\)

Code

// 40ms
    fun getDescentPeriods(p: IntArray) =
    p.indices.fold(0 to 0L) { (cnt, res), i ->
        val c = 1 + if (i > 0 && p[i] == p[i-1] - 1) cnt else 0
        c to (res + c)
    }.second

// 0ms
    pub fn get_descent_periods(p: Vec<i32>) -> i64 {
        (0..p.len()).fold((0,0), |(cnt,res), i| {
            let c = 1 + if i > 0 && p[i] == p[i-1]-1 { cnt } else { 0 };
            (c, res + c)
        }).1
    }