Online Stock Span

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.

思路：單調遞減棧，也就是進來一個數，跟前面的數比較，如果peek <= value，pop，用node記錄index和value，這樣可以省空間；不用存歷史記錄，只用記錄歷史的node和index即可；T: O(N); S: O(N)

class StockSpanner {
    private class Node {
        public int value;
        public int index;
        public Node(int index, int value) {
            this.index = index;
            this.value = value;
        }
    }

    private Stack<Node> stack;
    private int index;
    public StockSpanner() {
        stack = new Stack<Node>();
        index = 0;
    }
    
    public int next(int price) {
        Node node = new Node(++index, price);
        while(!stack.isEmpty() && stack.peek().value <= node.value) {
            stack.pop();
        }
        if(stack.isEmpty()) {
            stack.push(node);
            return node.index;
        } else {
            int res = node.index - stack.peek().index;
            stack.push(node);
            return res;
        }
    }
}

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