Daily Temperatures

Given a list of daily temperatures T, return a list such 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 of temperatures T = [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].

思路：就是单调栈，从后往前扫描，5, 3, 7那么从左往右看，5存在，3没必要存了，那么就踢掉；O(N); 也是找最左边和最右边的第一个最大值；也是单调递减栈；小的没必要存，踢掉比较小元素，记录 i  - stack.pop()的距离；

class Solution {
    public int[] dailyTemperatures(int[] A) {
        Stack<Integer> stack = new Stack<>();
        int n = A.length;
        int[] res = new int[n];
        for(int i = 0; i < n; i++) {
            if(stack.isEmpty()) {
                stack.push(i);
            } else {
                while(!stack.isEmpty() && A[stack.peek()] < A[i]) {
                    int j = stack.pop();
                    res[j] = i - j;
                }
                stack.push(i);
            }
        }
        return res;
    }
}