Use a sliding window approach where you maintain a window `[left, right]` and keep track of the current sum of the window. Expand the window by moving `right` and contract it by moving `left` when the sum exceeds or equals `target`.

Optimize the sliding window by using a prefix sum array to quickly calculate the sum of any subarray. This allows you to compute the sum of the current window in constant time.

Further optimize by avoiding the prefix sum array and instead maintaining a running sum of the current window. When the sum exceeds or equals `target`, try to shrink the window from the left to find the minimal length. This approach runs in O(n) time with O(1) space.