- `3) step` now apperas in the `while(counter == 0)` loop - shrink the left side of the window as long as counter == 0, until we break the `counter==0` balance. - time: O(n) one pass - space: O(1), ...

For example, ball < cat, dog < dorm, Happy < happy, Zoo < ball. A substring of a string is a contiguous block of characters in the string. For example, the substrings of abc are a, b, c, ab, bc, and ...