How do you find the next greater element for every item (Daily Temperatures)?

Q: How do you find the next greater element for every item (Daily Temperatures)?

**Target Audience:** Beginners preparing for coding interviews; you should know what a stack is. **Difficulty:** Medium **Language:** Python 3 > **How to read this doc:** Every idea is explained in plain language first, then tagged with `📌 Interview term:` so you also learn the exact words an interviewer expects. Every code block in this Python version was run, and the **Verified** section quotes the real output. ## 1. Why This Even Matters — A Story First You keep a diary of the temperature every day. For each day, you want to write next to it: *"how many days until it got warmer than this?"* The slow way is to take each day and read forward until you find a warmer one. On a long cold spell that means reading almost the whole diary again for every single day. A smarter diary-keeper reads the diary **once**. She keeps a short list of days that are "still waiting for a warmer day". When a new day arrives, every waiting day that is colder than it gets its answer right now (today min

The problem

Question presented to candidate: "Given a list of daily temperatures, return a list where each entry says how many days you have to wait after that day for a strictly warmer temperature, or 0 if there is none. Please code it in Python and get it under O(n squared)."

What a strong answer should cover:

The O(n²) brute force first, then the monotonic decreasing stack of indices that answers each day when a warmer day pops it.
Why it is O(n): each index is pushed once and popped at most once.
Strict < for strictly warmer, and the default 0 for days never popped.
The variants: circular (loop 2n with i % n) and the subset version with a hash map.
Python specifics: enumerate, [0] * n, a list as the stack with stack[-1], while stack and ..., and dict.get(x, -1) for the subset variant.

Clarifying questions expected:

"Is warmer strictly greater, or does an equal temperature count?"
"What should the answer be when no warmer day exists?"

Code / implementation expected: Yes — a Python function using a list of indices as a monotonic stack.

dsa
monotonic-stack
arrays

Try it yourself

Sketch your approach before you read the solution. That is what the interview tests.

Stuck? Get a hint.

A short nudge toward the approach. It will not give the answer away.

Solution

Target Audience: Beginners preparing for coding interviews; you should know what a stack is. Difficulty: Medium Language: Python 3

How to read this doc: Every idea is explained in plain language first, then tagged with 📌 Interview term: so you also learn the exact words an interviewer expects. Every code block in this Python version was run, and the Verified section quotes the real output.

1. Why This Even Matters — A Story First

You keep a diary of the temperature every day. For each day, you want to write next to it: "how many days until it got warmer than this?" The slow way is to take each day and read forward until you find a warmer one. On a long cold spell that means reading almost the whole diary again for every single day.

A smarter diary-keeper reads the diary once. She keeps a short list of days that are "still waiting for a warmer day". W

The solution is hidden so you can try first

6 min read · 11 sections

Python code

Edit the code and run it here. Each printed line with an expected value becomes a test.

PythonDaily Temperatures on the sample and edge cases

Practice

PythonBrute force vs monotonic stack: counting comparisons

Practice

PythonCircular next greater (LeetCode 503) and the hash-map subset (LeetCode 496)