mark occurances of elements in array cpp

1int sumOfDistinct(int a[], int n){
2  	int sum = 0; 
3    for (int i = 0; i < n; i++) { 
4      	 // If element appears first time 
5        if (a[abs(a[i]) - 1] >= 0) { 
6            sum += abs(a[i]); 
7            a[abs(a[i]) - 1] *= -1; 
8        } 
9    } 
10    return sum;