Prof. hardstone gives you an integer array $$$A$$$. The length of $$$A$$$ is $$$n$$$ and there are $$$m$$$ distinct numbers in $$$A$$$. Count the number of tuple $$$(l, r)$$$, $$$1 \leq l \leq r \leq n$$$, such that:
Numbers that appear in the interval $$$a[l...r]$$$ appear the same number of times.
For example, $$$A=[1,2,1,2]$$$, then there are $$$8$$$ legal tuples: $$$(1, 1), (2, 2), (3, 3), (4, 4), (1, 2), (2, 3), (3, 4), (1, 4)$$$.
This is an open problem with brain storm. $$$O(n^2m)$$$ brute force using the prefix sum and $$$O(n2^m)$$$ brute force using bitmasks and hashtable are easy to come up with. I am looking for a $$$O(nmlog^k)$$$ solution. Are there any smart data structures?
