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19. Daily Temperatures

mediumAsked at Flipkart

For each day report how many days until a warmer temperature — Flipkart uses it to test monotonic-stack fluency before harder pricing-window problems.

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

Problem

Given an array of integer temperatures representing daily temperatures, return an array answer where answer[i] is the number of days you have to wait after day i to get a warmer temperature. Use 0 when there is no future day with a warmer value.

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 scan

For each day, scan forward until you find a warmer 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

Keep a stack of indices waiting for a warmer day. When the current value exceeds the stack top, pop and record the distance. Each index pushed/popped once.

Time
O(n)
Space
O(n)
function dailyTemperatures(t) {
  const out = 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();
      out[j] = i - j;
    }
    stack.push(i);
  }
  return out;
}

Tradeoff:

Flipkart-specific tips

Flipkart screeners watch for whether you store indices or values on the stack — storing indices is the give-away that you have done this kind of problem before.

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Output

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