dp(mask, i) that returns the minimum XOR sum for a given bitmask mask and index i in nums1.i is equal to the length of nums1, return 0 as the base case.j in the bitmask that is set (indicating the element at index j in nums2 is not yet used), calculate the XOR of nums1[i] and nums2[j] and add it to the result of the recursive call dp(mask ^ (1 << j), i + 1).nums2 are available) and start the recursion from index 0 in nums1.dp.