Given two strings haystack and needle, return the index of the first occurrence of needle within haystack. If needle does not exist as a substring of haystack, return $-1$.

Constraints:

• $1 \leq$ haystack.length, needle.length $\leq 10^4$

• haystack and needle consist of only lowercase English characters

The key intuition behind this solution is to use a fixed-size sliding window of length equal to needle and slide it across haystack one character at a time, comparing the contents of each window against needle. As soon as a matching window is found, the starting index is returned immediately. If no match is found after exhausting all possible window positions, $-1$ is returned. This approach avoids any complex data structures and leverages direct substring comparison within each window.

Now, let’s look at the solution steps below:

1. Store the lengths of haystack and needle in haystackLen and needleLen, respectively.

2. Perform an early exit check if needleLen is greater than haystackLen, as needle cannot possibly be a substring. Return ...