21. Daily Temperatures

mediumAsked at Riot Games

For each day, find how many days until a warmer temperature — Riot uses the monotonic stack pattern when reasoning about next-event-time queries on server tick logs.

By Alex Chen, Founder, InterviewChamp.AI · Last verified 2026-05-20

Problem

Given an array of integers temperatures representing daily temperatures, return an array answer such that answer[i] is the number of days you have to wait after the ith day to get a warmer temperature. If none, answer[i] = 0.

Constraints

1 <= temperatures.length <= 10^5
30 <= temperatures[i] <= 100

Examples

Example 1

Input

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

Output

[1,1,4,2,1,1,0,0]

Example 2

Input

temperatures=[30,40,50,60]

Output

[1,1,1,0]

Approaches

1. Nested forward scan

For each index scan forward until a warmer day is found.

Time: O(n^2)
Space: O(1)

const ans = new Array(temperatures.length).fill(0);
for (let i=0;i<temperatures.length;i++) {
  for (let j=i+1;j<temperatures.length;j++) {
    if (temperatures[j]>temperatures[i]) { ans[i]=j-i; break; }
  }
}
return ans;

Tradeoff:

2. Monotonic decreasing stack

Push indices on a stack; pop when current temperature exceeds the top, recording the distance. Same monotonic stack Riot uses to compute next-event-time deltas across server tick streams in O(n).

Time: O(n)
Space: O(n)

function dailyTemperatures(t) {
  const ans = new Array(t.length).fill(0);
  const stack = [];
  for (let i=0;i<t.length;i++) {
    while (stack.length && t[i] > t[stack[stack.length-1]]) {
      const j = stack.pop();
      ans[j] = i - j;
    }
    stack.push(i);
  }
  return ans;
}

Tradeoff:

Riot Games-specific tips

Riot expects you to articulate why each index is pushed and popped exactly once — the same amortized analysis that justifies their monotonic event-queue traversals at server tick rate.

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