Remove duplicates from a linked list in single traversal

Given an unsorted linked list, write a function which deletes any duplicate nodes from the list by traversing the list only once.


For example,

Input:  5 -> 3 -> 4 -> 2 -> 5 -> 4 -> 1 -> 3 -> null
Output: 5 -> 3 -> 4 -> 2 -> 1 -> null

Simple solution would be to consider every distinct pair of nodes in the list and check if they have same data or not. If their data matches we delete the latter node. The time complexity of this solution is O(n2).

We can perform better by using hashing. The idea is to traverse the given list and insert each encountered node into a set. Now, if the current node already present in the set (i.e. it is seen before), we delete it and move to next element. At the end, we will have all duplicated removed from the list.


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5 -> 3 -> 4 -> 2 -> 1 -> null


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5 -> 3 -> 4 -> 2 -> 1 -> null


The time complexity of above solution is O(n) and auxiliary space used by the program is O(n).

Similar post: Remove duplicates from a sorted linked list

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I’m not going to take the bait, but I feel like the above algorithm can be written in five lines of python, which has built in hashed sets.


Look I code most of my code in C and don’t really have experience with C++ and the built-in hash tables,. But it seems silly to me that building a pseudo-copy of the original list follows the tenet of ‘in a single traversal’.

The only real way to do this is by getting a guaranteed sorted list from the start (or sorting it yourself, but that’s again not a single traversal).

I get that hash maps would be a good way to speed up comparisons; the task description just seems wrong to me though.