Feature: look back into data in a certain range in a shifting way.
// @k: size of sliding window
void sliding_window(const vector<int>& vec, const int k) {
int tot = 0; // total sum in the sliding window
for(int i=0; i<vec.size(); i++) {
/*
Do operation to window vec[max(i-k,0):i-1]
*/
tot += vec[i];
if(i>=k) tot -= vec[i-k];
/*
Do operation to window vec[max(i-k+1,0):i]
*/
}
}