class Solution {
    public int largestRectangleArea(int[] heights) {
        Stack<Integer> stack=new Stack<Integer>();
        int maxArea=0;
        for(int i=0;i<=heights.length;i++)
        {
            int h=i<heights.length?heights[i]:0;
            if(stack.isEmpty()||heights[stack.peek()]<=h)
                stack.push(i);
            
 
else
            {
                int idx=stack.pop();
                maxArea=Math.max(maxArea, heights[idx]*(stack.isEmpty()?i:i-stack.peek()-1));
                i--;
            }
        }
        return maxArea;
    }
}

84. Largest Rectangle in Histogram