// Largest Rectangle in Histogram — HARD

// Category: stack

Given an array of integers `heights` representing the histogram's bar heights (each bar has width `1`), return the **area of the largest rectangle** in the histogram.

**Constraints:**

- `1 <= heights.length <= 10⁵`

- `0 <= heights[i] <= 10⁴`

Example: heights = [2,1,5,6,2,3]

Output: 10