Minimum window substring
Grow the window until it has enough of every character of t, repeats included, then shrink it from the left while it still does. Counting how many of t's characters are matched makes "is it good?" O(1).
Grow the window until it has enough of every character of t, repeats included, then shrink it from the left while it still does. Counting how many of t's characters are matched makes "is it good?" O(1).