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21. Daily Temperatures

mediumAsked at LINE

For each day, find how many days until a warmer temperature — LINE uses this to gauge whether you reach for a monotonic stack, the same primitive behind their next-read-receipt scan.

By Alex Chen, Founder, InterviewChamp.AI · Last verified

Problem

Given an array temperatures of daily temperatures, return an array answer where 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 with a higher temperature, set answer[i] to 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. Brute force look-ahead

For each index, scan forward until you find a strictly larger value.

Time
O(n^2)
Space
O(1)
for(let i=0;i<n;i++){
  for(let j=i+1;j<n;j++)
    if(t[j]>t[i]){ out[i]=j-i; break; }
}

Tradeoff:

2. Monotonic decreasing stack

Walk left to right and keep a stack of indices with decreasing temperatures. Each time you see a hotter day, pop indices and record the gap. Each index enters and leaves the stack once.

Time
O(n)
Space
O(n)
function dailyTemperatures(temps) {
  const out = new Array(temps.length).fill(0);
  const stack = [];
  for (let i = 0; i < temps.length; i++) {
    while (stack.length && temps[i] > temps[stack[stack.length - 1]]) {
      const prev = stack.pop();
      out[prev] = i - prev;
    }
    stack.push(i);
  }
  return out;
}

Tradeoff:

LINE-specific tips

At LINE, link the monotonic-stack pattern to scanning a chat timeline for the next message with a read-receipt acknowledgement — presence + read-receipt framing wins.

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Output

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