There might be more than 1 missing numbers. Also, I suppose it is asking for O(1) space solution; otherwise, just use an array. If these are the cases, assume there is "no duplicate" in the linked list (and ignore non-positive number,) consider the followings: 1/ first pass => Get the size of the linked list, say, n 2/ second pass => Split list into two, A and B, while A get all data n/2 3/ If the size of A O(n)

(cont'd) 3/ If the size of A O(n)

(cont'd, again. not sure why the comment section messed up) 3/ If the size of A is less than n/2, the missing number must be in A. Split A like in the second step. 4/ If the size of A is equal to n/2, the missing number must be in B. Split B. Repeat 2 3 4 until base case. Running time would be n + n/2 + n/4 ... => O(n)