toArray method
Builds the final array from the difference array (prefix sum).
Implementation
List<int> toArray() {
final List<int> out = List<int>.filled(_d.length - 1, 0);
int s = 0;
for (int i = 0; i < out.length; i++) {
s += _d[i];
out[i] = s;
}
return out;
}