Implement strStr().

Return the index of the first occurrence of needle in haystack, or `-1` if `needle` is not part of `haystack`.

Clarification:

What should we return when `needle` is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when `needle` is an empty string. This is consistent to C's strstr() and Java's indexOf().

Example 1:

    Input: haystack = "hello", needle = "ll"
    Output: 2

Example 2:

    Input: haystack = "aaaaa", needle = "bba"
    Output: -1

Example 3:

    Input: haystack = "", needle = ""
    Output: 0

Constraints:

- `0 <= haystack.length, needle.length <= 5 * 104`
- `haystack` and `needle` consist of only lower-case English characters.