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LeetCode: Online Stock Span

aroup profile image Aroup Goldar Dhruba ・2 min read

Problem Statement

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:

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

Solution Thought Process

This problem can be modeled as a stack problem. For every element that we get, we try to put them into the stack. When putting on the stack while the stack is not empty, we try to destroy as many stocks as we can if the top of the stack is less than the value we are currently putting. We create a structure like this:

struct Stock {
    int val;
    int span;
};

On the span variable, we keep count of how many stocks we have destroyed with that stock. It has initially the value of 1. After that when we are destroying the stocks with some powerful stocks, we add the destroyed stock's span to the powerful stocks span.

Solution

class StockSpanner {
public:
    struct Stock {
        int val;
        int span;
    };
    stack<Stock>S;
public:
    StockSpanner() {

    }

    int next(int price) {
        Stock s{price,1};
        while(!S.empty())
        {
            Stock top = S.top();
            if(top.val<=s.val) {
                s.span+=top.span;
                S.pop();
            }
            else {
                break;
            }
        }
        S.push(s);
        return s.span;
    }
};

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

Complexity

Time Complexity: O(n)

Space Complexity: O(n)

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Aroup Goldar Dhruba

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