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Daily Temperatures

Medium
monotonic-stack

Given an array of integers temperatures represents the daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the i-th day to get a warmer temperature.

If there is no future day for which this is possible, keep answer[i] == 0 instead.

Key Insight: This is a classic "next greater element" problem where we need the distance instead of the value. Use a monotonic decreasing stack storing indices. When we pop an index, the distance to the warmer day is current_index - popped_index.

Example 1

Input: temperatures = [73,74,75,71,69,72,76,73]
Output: [1,1,4,2,1,1,0,0]
Explanation: Day 0 (73): wait 1 day for 74. Day 2 (75): wait 4 days for 76. Day 6-7: no warmer day.

Example 2

Input: temperatures = [30,40,50,60]
Output: [1,1,1,0]
Explanation: Each day (except last) waits exactly 1 day for the next warmer temperature.

Example 3

Input: temperatures = [30,60,90]
Output: [1,1,0]
Explanation: Strictly increasing temperatures mean each day waits 1 day (except the last).

Constraints

  • •1 <= temperatures.length <= 10^5
  • •30 <= temperatures[i] <= 100
Show Hints (4)
Hint 1: What information do we need when we find a warmer day? The index of the cooler day waiting.
Hint 2: A stack of indices (not temperatures) allows us to calculate waiting days.
Hint 3: When temperature[i] > temperature[stack.top()], we found a warmer day for stack.top().
Hint 4: The answer for popped index is i - poppedIndex.
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