LeetCode每日一题(901. Online Stock Span)

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该博客讨论了如何设计一个算法，该算法利用栈数据结构来计算股票价格的跨度。给定每天的股票价格，股票的跨度定义为从当天开始往回数的最大连续天数，在这段时间内股票价格小于或等于当天的价格。例如，如果股票价格在接下来的7天分别为[100, 80, 60, 70, 60, 75, 85]，则股票的跨度依次为[1, 1, 1, 2, 1, 4, 6]。实现了一个StockSpanner类，其中next方法接收当天股票价格并返回其跨度。

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.

用一个 stack 来记录 span，新进来的 price 与前面的 span 的 price 进行对比，如果 price >= last_span.price, 我们 pop stack 并且 last_span.count 加到当前的 span 中。如果 price < last_span.price, 则把当前 span push 到 stack 中


impl StockSpanner {
    fn new() -> Self {
        Self { l: Vec::new() }
    }

    fn next(&mut self, price: i32) -> i32 {
        let mut count = 1;
        while let Some((v, c)) = self.l.pop() {
            if price >= v {
                count += c;
                continue;
            }
            self.l.push((v, c));
            break;
        }
        self.l.push((price, count));
        count
    }
}