main.cpp

// Source: https://leetcode.com/problems/number-of-smooth-descent-periods-of-a-stock
// Title: Number of Smooth Descent Periods of a Stock
// Difficulty: Medium
// Author: Mu Yang <http://muyang.pro>

////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////
// You are given an integer array `prices` representing the daily price history of a stock, where `prices[i]` is the stock price on the `i^th` day.
//
// A **smooth descent period** of a stock consists of **one or more contiguous** days such that the price on each day is **lower** than the price on the **preceding day** by **exactly** `1`. The first day of the period is exempted from this rule.
//
// Return the number of **smooth descent periods**.
//
// **Example 1:**
//
// ```
// Input: prices = [3,2,1,4]
// Output: 7
// Explanation: There are 7 smooth descent periods:
// [3], [2], [1], [4], [3,2], [2,1], and [3,2,1]
// Note that a period with one day is a smooth descent period by the definition.
// ```
//
// **Example 2:**
//
// ```
// Input: prices = [8,6,7,7]
// Output: 4
// Explanation: There are 4 smooth descent periods: [8], [6], [7], and [7]
// Note that [8,6] is not a smooth descent period as 8 - 6 ≠ 1.
// ```
//
// **Example 3:**
//
// ```
// Input: prices = [1]
// Output: 1
// Explanation: There is 1 smooth descent period: [1]
// ```
//
// **Constraints:**
//
// - `1 <= prices.length <= 10^5`
// - `1 <= prices[i] <= 10^5`
//
////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////////

#include <vector>

using namespace std;

class Solution {
 public:
  long long getDescentPeriods(vector<int>& prices) {
    auto ans = 0LL;
    auto size = 0;
    auto prev = -1;
    for (auto price : prices) {
      if (prev == price + 1) {
        ++size;
      } else {
        size = 1;
      }
      ans += size;
      prev = price;
    }

    return ans;
  }
};