Counting unique element in large array

Question

One of my colleagues was asked this question in an interview.

Given a huge array which stores unsigned int. Length of array is 100000000. Find the effective

Answer 1

If the range of the int values is limited, then you may allocate an array, which serves to count the occurrences for each possible value. Then you just iterate through your huge array and increment the counters.

foreach x in huge_array {
   counter[x]++;
}

Thus you find the solution in linear time (O(n)), but at the expense of memory consumption. That is, if your ints span the whole range allowed by 32-bit ints, you would need to allocate an array of 4G ints, which is impractical...