You are given array of integers, your task will be to count all pairs in that array and return their count.
Notes:
- Array can be empty or contain only one value; in this case return
0 - If there are more pairs of a certain number, count each pair only once. E.g.: for
[0, 0, 0, 0]the return value is2(= 2 pairs of0s) - Random tests: maximum array length is 1000, range of values in array is between 0 and 1000
[1, 2, 5, 6, 5, 2] --> 2
...because there are 2 pairs: 2 and 5
[1, 2, 2, 20, 6, 20, 2, 6, 2] --> 4
...because there are 4 pairs: 2, 20, 6 and 2 (again)