Given a circular integer array nums of length n , return the maximum possible sum of a non-empty subarray of nums .
A circular array means the end of the array connects to the beginning of the array. Formally, the next element of nums[i] is nums[(i + 1) % n] and the previous element of nums[i] is nums[(i - 1 + n) % n] .
A subarray may only include each element of the fixed buffer nums at most once. Formally, for a subarray nums[i], nums[i + 1], ..., nums[j] , there does not exist i <= k1 , k2 <= j with k1 % n == k2 % n .
n == nums.length1 <= n <= 3 * 10 ^4-3 * 10 ^4 <= nums[i] <= 3 * 10 ^4