84\. Largest Rectangle in Histogram

Hard

Given an array of integers `heights` representing the histogram's bar height where the width of each bar is `1`, return _the area of the largest rectangle in the histogram_.

**Example 1:**

![](https://assets.leetcode.com/uploads/2021/01/04/histogram.jpg)

**Input:** heights = [2,1,5,6,2,3]

**Output:** 10

**Explanation:** The above is a histogram where width of each bar is 1. The largest rectangle is shown in the red area, which has an area = 10 units. 

**Example 2:**

![](https://assets.leetcode.com/uploads/2021/01/04/histogram-1.jpg)

**Input:** heights = [2,4]

**Output:** 4 

**Constraints:**

*   <code>1 <= heights.length <= 10<sup>5</sup></code>
*   <code>0 <= heights[i] <= 10<sup>4</sup></code>