Medium: Subarray with a given XOR

Use reversible XOR

Let prefix be XOR of elements through the current index. A subarray ending here has XOR target when earlierPrefix xor prefix = target, equivalently earlierPrefix = prefix xor target. Keep a frequency map of prefixes already seen. For every new value, update prefix, add count[prefix xor target] to the answer, then increment count[prefix]. Seed the map with {0: 1} so a prefix itself equal to target is counted.

Explain ordering

Query before inserting the current prefix so a nonempty subarray is counted and the current position is not paired with itself. XOR is associative and self-inverse, which is why the transformation works even when numbers are negative under fixed-width bit representation. A nested start/end scan is O(n^2); this method is O(n) expected time and O(n) space. Use a wide answer type because the number of qualifying subarrays can be quadratic.