Given two strings haystack and needle, return the index of the first occurrence of needle in haystack, or -1.
Example 1:
Input: haystack = "sadbutsad", needle = "sad"
Output: 0
Explanation: "sad" occurs at index 0 (and 6), return the first occurrence.
Example 2:
Input: haystack = "leetcode", needle = "leeto"
Output: -1
Explanation: "leeto" is not found in "leetcode".