LeetCode: 901. Online Stock Span

解题思路

Write a class StockSpanner which 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 backwards) for which the price of the stock 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].

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: 
First, S = StockSpanner() is initialized.  Then:
S.next(100) is called and returns 1,
S.next(80) is called and returns 1,
S.next(60) is called and returns 1,
S.next(70) is called and returns 2,
S.next(60) is called and returns 1,
S.next(75) is called and returns 4,
S.next(85) is called and returns 6.

Note that (for example) S.next(75) returned 4, because the last 4 prices
(including today's price of 75) were less than or equal to today's price.

Note:

Calls to StockSpanner.next(int price) will have 1 <= price <= 10^5.
There will be at most 10000 calls to StockSpanner.next per test case.
There will be at most 150000 calls to StockSpanner.next across all test cases.
The total time limit for this problem has been reduced by 75% for C++, 
and 50% for all other languages.

解题思路

用数组记录下已计算过的 span 值，每次调用 next 时利用之前记录的 span 值计算新加入的元素的 span 值。

AC 代码

class StockSpanner {
public:
    StockSpanner() {

    }

    int next(int price) {
        int cnt = 0;
        m_prices.push_back(price);
        m_record.push_back(1);

        for(int i = m_prices.size()-1; i >= 0; )
        {
            if(m_prices[i] > price) break;
            cnt += m_record[i];
            i -= m_record[i];
        }

        m_record.back() = cnt;
        return cnt;
    }

private:
    vector<int> m_record; // 记录上一天的 span
    vector<int> m_prices; // 记录每天的价格
};

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