# 0901. Online Stock Span

https://leetcode.com/problems/online-stock-span

## Description

Design an algorithm that collects daily price quotes for some stock and returns **the span** of that stock's price for the current day.

The **span** of the stock's price today is defined as the maximum number of consecutive days (starting from today and going backward) for which the stock price was less than or equal to today's price.

* For example, if the price of a stock over the next `7` days were `[100,80,60,70,60,75,85]`, then the stock spans would be `[1,1,1,2,1,4,6]`.

Implement the `StockSpanner` class:

* `StockSpanner()` Initializes the object of the class.
* `int next(int price)` Returns the **span** of the stock's price given that today's price is `price`.

**Example 1:**

```
**Input**
["StockSpanner", "next", "next", "next", "next", "next", "next", "next"]
[[], [100], [80], [60], [70], [60], [75], [85]]
**Output**
[null, 1, 1, 1, 2, 1, 4, 6]
**Explanation**
StockSpanner stockSpanner = new StockSpanner();
stockSpanner.next(100); // return 1
stockSpanner.next(80);  // return 1
stockSpanner.next(60);  // return 1
stockSpanner.next(70);  // return 2
stockSpanner.next(60);  // return 1
stockSpanner.next(75);  // return 4, because the last 4 prices (including today's price of 75) were less than or equal to today's price.
stockSpanner.next(85);  // return 6
```

**Constraints:**

* `1 <= price <= 105`
* At most `104` calls will be made to `next`.

## ac: Monotonic queue

Intuitive version:

```java
class StockSpanner {
    Deque<int[]> stack; // [day, price]
    int day;
    
    public StockSpanner() {
        stack = new ArrayDeque<>();
        day = 0;
    }
    
    public int next(int price) {
        while (!stack.isEmpty() && stack.peek()[1] <= price) {
            stack.pop();
        }
        
        int span = 1;
        if (stack.isEmpty()) {
            span = Math.max(span, day + 1);
        } else {
            int[] stock = stack.peek();
            span = Math.max(span, day - stock[0]);
        }

        stack.push(new int[]{day, price});
        day++;
        
        return span;
    }
}

/**
 * Your StockSpanner object will be instantiated and called as such:
 * StockSpanner obj = new StockSpanner();
 * int param_1 = obj.next(price);
 */
```

This is more concise:

```java
class StockSpanner {
    Deque<int[]> stack; // [span, price]
    
    public StockSpanner() {
        stack = new ArrayDeque<>();
    }
    
    public int next(int price) {
        int span = 1;
        while (!stack.isEmpty() && stack.peek()[1] <= price) {
            span += stack.pop()[0]; // Sum up span of previous smaller price.
        }

        stack.push(new int[]{span, price});
        
        return span;
    }
}
```
