leetcode 2444. Count Subarrays With Fixed Bounds - two/there-pointer sliding window method

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Python solution to Leetcode problem 2444: Count Subarrays With Fixed Bounds. The idea is simple: for each index i, we find the number of valid fixed-bound subarrays ending there. Summing up these contributions, we get the return. This corresponds to the 1st version of solution format, iterating the nums list from beginning to the end. Symmetrically, we could do the implementation from end to front, and this corresponds to the 2nd soution format.