Next Greater Element (Monotonic Stack)

For each element, find the next element to its right that's strictly greater.

Implement:

• next_greater(nums): a list where entry i is the next greater value, or -1
• daily_temperatures(temps): for each day, how many days until a warmer one (0 if never)
• next_greater_circular(nums): like next_greater, but the array wraps around

All three should run in O(n) using a stack of indices.