Perfect solution to http://codility.com/demo/take-sample-test/

equilibrium.cpp

// This solution scored 100/100
int equi ( const vector<int> &A ) {
    // write your code here
    int size = A.size();
    if (size == 0) return -1;
     
    long long *c_left = new long long[size];
    
    c_left[0] = A[0];
    for(int i = 1; i < size; i++) {
        c_left[i] = c_left[i-1] + A[i];
    }

    long long c_right = c_left[size-1]-A[0];

    if (c_right == 0) return 0;

    for (int k = 1; k < size; k++) {
        c_right = c_right - A[k];
	
        if (c_left[k-1] == c_right) return k;            
    }

    return -1;
}