Leetcode 84: Largest Rectangle in Histogram

Given n non-negative integers representing the histogram's bar height where the width of each bar is 1, find the area of largest rectangle in the histogram.

Above is a histogram where width of each bar is 1, given height = [2,1,5,6,2,3].

 

The largest rectangle is shown in the shaded area, which has area = 10 unit.

 

For example,
Given heights = [2,1,5,6,2,3],
return 10.

 

Note: this problem is pretty tricky, I could not solve it by myself in O(n) time.

public class Solution {
    public int LargestRectangleArea(int[] heights) {
        Stack<int> stack = new Stack<int>();
        int i = 0, maxArea = 0;
             
        while (i <= heights.Length)
        {
            var h = i == heights.Length ? 0 : heights[i];
            if (stack.Count == 0 || h >= heights[stack.Peek()])
            {
                stack.Push(i);
                i++;
            }
            else
            {
                var left = stack.Pop();
                
                maxArea = Math.Max(maxArea, stack.Count == 0 ? heights[left] * i : heights[left] * (i - stack.Peek() - 1));
            }
        }
        
        return maxArea;
    }
}