Leetcode 739. Daily Temperatures

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].

Example

Example 1:
	Input:  temperatures = [73, 74, 75, 71, 69, 72, 76, 73]
	Output:  [1, 1, 4, 2, 1, 1, 0, 0]
	
	Explanation:
	Just find the first day after it which has higher temperatures than it.

	
Example 2:
	Input: temperatures = [50, 40, 39, 30]
	Output:  [0,0,0,0]
	

Notice

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

Code (Java):

class Solution {
    public int[] dailyTemperatures(int[] T) {
        if (T == null || T.length == 0) {
            return new int[0];
        }
        
        int[] ans = new int[T.length];
        
        // monotonic descreasing stack
        Stack<Integer> stack = new Stack<>();
        
        for (int i = T.length - 1; i >= 0; i--) {
            while (!stack.isEmpty() && T[i] >= T[stack.peek()]) {
                stack.pop();
            }
            
            ans[i] = stack.isEmpty() ? 0 : stack.peek() - i;
            
            stack.push(i);
        }
        
        return ans;
    }
}