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COMPUTE number of inversion in an array - ArrayInversionCount

Task description
A zero-indexed array A consisting of N integers is given. An inversionis 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); }
thatCOMPUTES the number of inversions in A, or returns −1 if it exceeds 1,000,000,000.
Assume that:
  • 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].
For example, in the following array:
A[0] = -1 A[1] = 6 A[2] = 3 A[3] = 4 A[4] = 7 A[5] = 4there are four inversions:
(1,2) (1,3) (1,5) (4,5)so the function should return 4.
Complexity:
  • expected worst-case time complexity is O(N*log(N));
  • expected worst-case space complexity is O(N), beyond input storage (not counting the storage required for input arguments).
Elements of input arrays can be modified.

// you can also use imports, for example: // import java.util.*; // you can write to stdout for debugging purposes, e.g. // System.out.println("this is a debug message"); import java.math.BigDecimal; class Solution { public int solution(int[] A) { // write your code in Java SE 8 int len=A.length; int count=0; int i=0; int j=0; while(i<len){ j=i+1; while(j<len){ if(A[i]>A[j]){ count++; } j++; } i++; } if(count>1000000000) count=-1; return count; } }


Analysis summary
The following issues have been detected: timeout errors.
Analysis
Detected time complexity:
O(N**2)
expand allExample tests
▶
example1 
example test
✔
OK
expand allCorrectness tests
▶
simple1
✔
OK
▶
simple2
✔
OK
▶
simple3
✔
OK
▶
extreme_0_inv 
[0], [], [1,2,3], [1,1,1]
✔
OK
▶
medium1 
n=100
✔
OK
▶
medium2 
n=200
✔
OK
expand allPerformance tests
▶
medium3 
n=1000
✔
OK
▶
big1 
n=10000
✔
OK
▶
big2 
n=20000
✘
TIMEOUT ERROR 
running time: 5.18 sec., time limit: 2.71 sec.
▶
big3 
n=30000
✘
TIMEOUT ERROR 
running time: >8.00 sec., time limit: 2.85 sec.
▶
big_monotonic 
long descending and non-ascending sequence
✘
TIMEOUT ERROR 
running time: >8.00 sec., time limit: 2.97 sec.

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