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 contain {1,2,3,4}
The solution can be achieved using two for loops but that would be O(n^2) I believe.

Answer 1

If, and only if, the source array is sorted, this can be done in linear time:

std::unique(a, a + 5); //Returns a pointer to the new logical end of a.

Otherwise you’ll have to sort first, which is (99.999% of the time) n lg n.