Leetcode 901. Online Stock Span （单调栈经典题）

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本文介绍如何设计一个计算股票价格跨度的算法，利用单调栈优化查询效率。给出StockSpanner类实例和测试用例。

Online Stock Span
Medium
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 in one day is the maximum number of consecutive days (starting from that day and going backward) for which the stock price was less than or equal to the price of that day.

For example, if the prices of the stock in the last four days is [7,2,1,2] and the price of the stock today is 2, then the span of today is 4 because starting from today, the price of the stock was less than or equal 2 for 4 consecutive days.
Also, if the prices of the stock in the last four days is [7,34,1,2] and the price of the stock today is 8, then the span of today is 3 because starting from today, the price of the stock was less than or equal 8 for 3 consecutive days.
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.

解法1：单调栈

class StockSpanner {
public:
    StockSpanner() : index(0) {
        
    }
    
    int next(int price) {
        int res = 0;
        while (!stk.empty() && data[stk.top()] <= price) {
            stk.pop();
        }
        if (stk.empty()) res = index + 1;
        else res = index - stk.top();
        data.push_back(price);
        stk.push(index);
        index++;
        return res;
    }
private:
    vector<int> data;
    stack<int> stk;
    int index;
};

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