Nice puzzle! I think you can allow both kinds of deletions while still using four pointers per link node: * prev1 XOR next1 * prev2 XOR next2 * prev1 XOR pre

Nice puzzle! I think you can allow both kinds of deletions while still using four pointers per link node: * prev1 XOR next1 * prev2 XOR next2 * prev1 XOR prev2 * data1 XOR data2 Because while traversing you always know one of {prev1, prev2} and one of {data1, data2}. Right?