How does one remove duplicate elements in place in an array in O(n) in C or C++?

Question

Is there any method to remove the duplicate elements in an array in place in C/C++ in O(n)? Suppose elements are a[5]={1,2,2,3,4} then resulting array should c

Answer 1

Yes. Because access (insertion or lookup) on a hashtable is O(1), you can remove duplicates in O(N).

Pseudocode:

hashtable h = {}
numdups = 0
for (i = 0; i < input.length; i++) {
    if (!h.contains(input[i])) {
        input[i-numdups] = input[i]
        h.add(input[i])
    } else {
        numdups = numdups + 1
    }

This is O(N).

Some commenters have pointed out that whether a hashtable is O(1) depends on a number of things. But in the real world, with a good hash, you can expect constant-time performance. And it is possible to engineer a hash that is O(1) to satisfy the theoreticians.