https://www.hackerrank.com/challenges/ctci-array-left-rotationy
// Complete the rotLeft function below.
vector<int> rotLeft(vector<int> a, int d) {
d = d % a.size();
for (int i = 0; i < d; ++i) {
a.push_back(a.front());
a.erase(a.begin());
}
return a;
}