Daily Temperatures

Question

Given a list of daily temperatures, produce a list that, for each day in the input, tells you how many days you would have to wait until a warmer temperature. If there is no future day for which this is possible, put 0 instead.

For example, given the list temperatures = [73, 74, 75, 71, 69, 72, 76, 73], your output should be [1, 1, 4, 2, 1, 1, 0, 0].

Note: The length of temperatures will be in the range [1, 30000]. Each temperature will be an integer in the range [30, 100].

Solution

自己參加 contest 的時候寫不出來，用了一個很 naive 的方式但得到了 TLE。看了解答之後發現，題目要求的是第一個比自己大的數在哪裡，因此可以套用 Largest Rectangle in Histogram 的方法，用一個 “遞減 stack”（概念跟遞增 stack 相同） 來達到此目的。

具體概念是說，我們只要遇到比 temperatures[stack.peek()] 小的數，就存進 stack 裡面，當遇到比較大的數，就 stack.pop 並用兩個 index 相減，就可以得到相差的天數

    public int[] dailyTemperatures(int[] temperatures) {
        if (temperatures == null || temperatures.length == 0) {
            return null;
        }
        int len = temperatures.length;
        int[] result = new int[len];
        Stack<Integer> stack = new Stack<Integer>();
        for (int i = 0; i < len; i++) {
            while (!stack.isEmpty() && temperatures[i] > temperatures[stack.peek()]) {
                int index = stack.pop();
                result[index] = i - index;
            }

            stack.push(i);
        }
        return result;
    }