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Compute number of inversion in an array.

An array A consisting of N integers is given. An inversion is a pair of indexes (P, Q) such that P < Q and A[Q] < A[P].

Write a function:

class Solution { public int solution(int[] A); }

that computes the number of inversions in A, or returns −1 if it exceeds 1,000,000,000.

For example, in the following array:

A[0] = -1 A[1] = 6 A[2] = 3 A[3] = 4 A[4] = 7 A[5] = 4

there are four inversions:

(1,2) (1,3) (1,5) (4,5)

so the function should return 4.

Write an efficient algorithm for the following assumptions:

  • N is an integer within the range [0..100,000];
  • each element of array A is an integer within the range [−2,147,483,648..2,147,483,647].
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