Given two strings haystack and needle, return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
Example 1:
Input: haystack = "sadbutsad", needle = "sad" Output: 0
Example 2:
Input: haystack = "leetcode", needle = "leeto" Output: -1
Constraints:
1 <= haystack.length, needle.length <= 104