Add algorithm: single_number using XOR

maths/special_numbers/single_number.py

@@ -0,0 +1,18 @@
+def single_number(nums: list[int]) -> int:
+    """
+    Find the element that appears only once in a list
+    where every other element appears twice.
+
+    This algorithm uses bitwise XOR to cancel out duplicate numbers.
+
+    >>> single_number([2, 2, 1])
+    1
+    >>> single_number([4, 1, 2, 1, 2])
+    4
+    >>> single_number([1])
+    1
+    """
+    res = 0
+    for n in nums:
+        res ^= n
+    return res