I'm recently thinking of a problem. Given a sequence $$$a$$$, find the value of $$$\sum_{l=1}^n \sum_{r=l+1}^n lis(a[l...r])$$$. Where $$$lis(a)=$$$ the longest non-decreasing sequence of $$$a$$$ and $$$n$$$ is the length of $$$a$$$. $$$n$$$ is large to $$$10^5$$$ and $$$a_i$$$ is extremely small, only $$$20$$$? I can't think of a solution. Can anyone help me? Please.