## What is it? Given an array of daily temperatures, find how many days until a warmer temperature for each day. If no warmer day exists in the future, output 0. ## How it works - Use a monotonic decreasing stack storing indices - For each day `i`: - While `temperatures[i] > temperatures[stack.top()]` → the wait for the top index is `i - stack.top()` days; pop - Push `i` - Indices remaining on the stack have no warmer future day → their answer is 0 ## When to use - "How many steps until condition X is satisfied" for each element - Any problem involving nearest future element satisfying a monotone condition - Classic interview problem demonstrating monotonic stack ## Key Points - O(n) time — each index pushed and popped once - O(n) space for the stack - The answer array is filled backwards as greater elements are found - Great example of converting O(n²) brute force to O(n) with a stack