import java.util.*;

public class Solution {

  public static int largestRectangle(ArrayList < Integer > heights) {

        int len = heights.size();

        int rb[] = new int [len];

        Stack<Integer> st = new Stack<>();

        // calculating right boundary

        for (int i = len-1; i >= 0; i--) {

            int cur = heights.get(i);

            while (!st.isEmpty() && heights.get(st.peek()) >= cur)

                st.pop();

            if (st.isEmpty() == true)

                rb[i] = len;

            else

                rb[i] = st.peek();

            st.push(i);

        }

        int lb[] = new int [len];

        st = new Stack<>();

        // calculating left boundary

        for (int i = 0; i < len; i++) {

            int cur = heights.get(i);

            while (!st.isEmpty() && heights.get(st.peek()) >= cur)

                st.pop();

            if (st.isEmpty() == true)

                lb[i] = -1;

            else

                lb[i] = st.peek();

            st.push(i);

        }

        // calcution of max Area

        int maxArea = 0;

        for (int i = 0; i < len; i++) {

            int width = rb[i] - lb[i] - 1;

            int curArea = width * heights.get(i);

            maxArea = Math.max(maxArea, curArea);

        }

        return maxArea;

  }

}