Leetcode 3395. Subsequences with a Unique Middle Mode I

Count the number of length-5 subsequences whose middle element is the unique mode (i.e., appears strictly more times than any other value in the five), returning the result modulo 1e9+7. The core challenge is combinatorial counting for each index as the middle—using prefix/suffix frequencies to count ways to pick two left and two right elements so the middle value’s frequency strictly exceeds all others.