901. Online Stock Span

Problem

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.

Problem Summary

Write a StockSpanner class that collects daily price quotes for a stock and returns the span of that stock's price for the current day.

Today's stock price span is defined as the maximum number of consecutive days for which the stock price was less than or equal to today's price (counting backward from today, including today).

For example, if the stock prices over the next 7 days are [100, 80, 60, 70, 60, 75, 85], then the stock spans will be [1, 1, 1, 2, 1, 4, 6].

Solution Approach

This problem is a monotonic stack problem. Maintain indices in monotonically increasing order.

Summary

Similar monotonic stack problems:

496. Next Greater Element I
497. Next Greater Element II
498. Daily Temperatures
499. Sum of Subarray Minimums
500. Largest Rectangle in Histogram

Code

package leetcode

import "fmt"

// node pair
type Node struct {
	Val int
	res int
}

// slice
type StockSpanner struct {
	Item []Node
}

func Constructor901() StockSpanner {
	stockSpanner := StockSpanner{make([]Node, 0)}
	return stockSpanner
}

// need refactor later
func (this *StockSpanner) Next(price int) int {
	res := 1
	if len(this.Item) == 0 {
		this.Item = append(this.Item, Node{price, res})
		return res
	}
	for len(this.Item) > 0 && this.Item[len(this.Item)-1].Val <= price {
		res = res + this.Item[len(this.Item)-1].res
		this.Item = this.Item[:len(this.Item)-1]
	}
	this.Item = append(this.Item, Node{price, res})
	fmt.Printf("this.Item = %v\n", this.Item)
	return res
}

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