LeetCode Challenge Day 91 — 2110. Number of Smooth Descent Periods of a Stock

Hey folks 👋

This is Day 91 of my LeetCode streak 🚀
Today's problem is 2110. Number of Smooth Descent Periods of a Stock — a clean problem where consistency in thinking beats brute force.

📌 Problem Statement

You are given an integer array prices, where prices[i] represents the stock price on the i-th day.

A smooth descent period:

• Consists of one or more contiguous days
• Each day's price is exactly 1 less than the previous day
• A single day always counts as a valid period

Goal:
Return the total number of smooth descent periods.

💡 Intuition

Every individual day forms a valid smooth descent period.

The important observation is:

A smooth descent only continues when the price drops by exactly 1.

So instead of checking all possible subarrays, we track:

• The length of the current descending streak
• How many valid periods end at each day

If a streak has length k, it contributes k smooth descent periods ending at that position.

🔑 Approach

1. Initialize:
   • curr = 1 → current descent streak length
   • ans = 1 → the first day itself
2. Traverse the array from index 1:
   • If prices[i - 1] - prices[i] === 1, increment curr
   • Otherwise, reset curr to 1
3. Add curr to ans at each step
4. Return ans after completing the traversal

This ensures all valid periods are counted efficiently in one pass.

⏱️ Complexity Analysis

• Time Complexity:
  O(n) — single pass through the array

• Space Complexity:
  O(1) — constant extra space

🧑‍💻 Code (JavaScript)

/**
 * @param {number[]} prices
 * @return {number}
 */
var getDescentPeriods = function(prices) {
    let ans = 1;       // first day itself
    let curr = 1;      // current descent length

    for (let i = 1; i < prices.length; i++) {
        if (prices[i - 1] - prices[i] === 1) {
            curr += 1;     // extend descent
        } else {
            curr = 1;      // reset
        }
        ans += curr;
    }

    return ans;
};

🎯 Reflection

This problem reinforces a core lesson:

• Track what ends at the current index

• Let patterns accumulate naturally

• One-pass solutions are often hiding in plain sight

That wraps up Day 91 of my LeetCode challenge 🔥
Staying consistent, one problem at a time 🚀

Happy Coding 👨‍💻