Maximum_Rectangle_Size_Histogram

def largestRectangleArea(heights):
    leftSmall = []
    rightSmall = [-1]*len(heights)
    stack = []
    for i in range(len(heights)):
        while(stack and heights[i] <= stack[-1][1]):
            stack.pop()
        if(stack):
            leftSmall.append(stack[-1][0]+1)
        else:
            leftSmall.append(0)
        stack.append([i,heights[i]])
    stack = []
    for i in range(len(heights)-1, -1, -1):
        while(stack and heights[i] <= stack[-1][1]):
            stack.pop()
        if(stack):
            rightSmall[i] = stack[-1][0]-1
        else:
            rightSmall[i] = len(heights)-1
        stack.append([i, heights[i]])
    res = 0
    for i in range(len(heights)):
        val = (rightSmall[i]-leftSmall[i]+1)*heights[i]
        res = max(res, val)
    return res

heights = [2, 1, 5, 6, 2, 3, 1]
print(largestRectangleArea(heights))