kt: add «1863. Sum of All Subset XOR Totals»

Signed-off-by: Matej Focko <me@mfocko.xyz>

kt/sum-of-all-subset-xor-totals.kt

@@ -0,0 +1,18 @@
+class Solution {
+    private fun xorSum(
+        nums: IntArray,
+        i: Int,
+        sum: Int,
+    ): Int {
+        if (i == nums.size) {
+            return sum
+        }
+
+        val including = xorSum(nums, i + 1, sum xor nums[i])
+        val excluding = xorSum(nums, i + 1, sum)
+
+        return including + excluding
+    }
+
+    fun subsetXORSum(nums: IntArray): Int = xorSum(nums, 0, 0)
+}