What is the brute force approach and complexity for Daily Temperatures?

For each day, walk forward until a strictly warmer day with time complexity O(N²) and space complexity O(1).

What is the optimal approach and complexity for Daily Temperatures?

One pass; a warmer day pops every cooler waiting day and writes its distance with optimal time complexity O(N) and space complexity O(N).

How do you identify the pattern for Daily Temperatures?

Look for key signals in the prompt: "until a warmer day" indicates The first bigger value to the right → next greater element, decreasing stack.; "how many days you have to wait" indicates The answer is a distance, so the stack must store **indices**.; "strictly warmer" indicates Pop with `<`, not `<=`; an equal day keeps waiting..

Daily Temperatures

Medium·LeetCode #739 Pro OnlyBasic Reinforcement

LeetCode

Problem Statement

You get the daily temperatures as an array temperatures, one value per day.

For each day, find out how many days you have to wait until a strictly warmer day comes. If no warmer day ever comes, the answer for that day is 0.

Input

An integer array temperatures.

Output

An array answer of the same length, where answer[i] is the number of days after day i until a warmer temperature, or 0.

Example 1

Input: temperatures = [73, 74, 75, 71, 69, 72, 76, 73]

Output: [1, 1, 4, 2, 1, 1, 0, 0]

Day 2 (75°) waits until day 6 (76°): 4 days. Day 3 (71°) waits until day 5 (72°): 2 days. Days 6 and 7 never see a warmer day.

Example 2

Input: temperatures = [30, 40, 50, 60]

Output: [1, 1, 1, 0]

Every day is followed directly by a warmer one, except the last.

Example 3

Input: temperatures = [30, 60, 90]

Output: [1, 1, 0]

Constraints

• 1 ≤ temperatures.length ≤ 10^5
• 30 ≤ temperatures[i] ≤ 100

Pattern Recognition Clues

Recognising these key signals in the problem description immediately identifies the pattern:

“until a warmer day”

→ The first bigger value to the right → next greater element, decreasing stack.

“how many days you have to wait”

→ The answer is a distance, so the stack must store **indices**.

“strictly warmer”

→ Pop with `<`, not `<=`; an equal day keeps waiting.

Approach Complexity Ladder

Brute Force — scan ahead from every dayO(N²)O(1)

For each day, walk forward until a strictly warmer day

Optimal — stack of waiting days (indices)O(N)O(N)

One pass; a warmer day pops every cooler waiting day and writes its distance

Brute Force Intuition

For each day i, look at days i+1, i+2, … until one is strictly warmer. The answer is the distance j − i. If you reach the end, it stays 0.

Recognition rule for this sub-pattern:

Need the first bigger value to the right (or left). Keep a decreasing stack; a bigger arrival resolves everyone smaller.

Remember this · Monotonic Stack

Trigger

Next/previous greater or smaller element, spans, histogram areas, "for each element, how far does it dominate?"

Think

When a new element arrives, which older elements can never be the answer again?

Key question

Is my stack increasing or decreasing, and what do I record when I pop?

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