algorithm: find count of numbers within a given range

given an unsorted number array where there can be duplicates, pre-process the array so that to find the count of numbers within a given range, the time is O(1).

For example, 7,2,3,2,4,1,4,6. The count of numbers both >= 2 and <= 5 is 5. (2,2,3,4,4).