Codility PermCheck why my solution is not working

The reason this isn't working is that a permutation (as explained) is not the only way to arrive at a particular sum, as your solution assumes. For example:

[0, 1, 2, 3] // Sum == 6
[0, 2, 2, 2] // Sum == 6

According to the problem description as written, I don't believe it implies the given data has no duplicates.

If N is 100,000, then the N * (N + 1) / 2 expression causes integer overflow(eventhough sum is a long, N is an int). Not sure if there are other bugs.

Code: 08:25:58 UTC, c, final, score: 100.00

   int solution(int A[], int N) {
    // write your code in C99

    int T[N];
//when the compiler sees N variable declaration it cannot know how many         
//elements there are in the array.
//must manually initialize that array.
    memset( T, 0, N*sizeof(int) ); 
    for (int i=0;i<N;i++)
    {
    if (A[i]>N)
        {
        return 0;
        }
    T[A[i]-1]++;
    }
    int sum=0;
   for (int i=0;i<N;i++)
    {
    sum =sum+T[A[i]-1];
    }
    return (sum==N)?1:0;
}