sliding window

two pointers

finding maximum length in array without repeats

maintain $s t a r t$ and $e n d$ pointers
increment $e n d$ while adding values to a set to check for repeats
when repeat is found, update maximum length
then, increment $s t a r t$ until clear of repeat

O(N) sliding window min

modifying deque to store minimums

deque<int> wq;
 
int winmin() {
  return wq.front();
}
 
void winadd(int x) {
    while (!wq.empty() && wq.back() > x) wq.pop_back();
    wq.push_back(x);
}
 
void winrm(int x) {
    if (!wq.empty() && wq.front() == x) wq.pop_front();
}

move across array while inserting $a_{i}$ and removing $a_{i - k}$ and take minimum of each window