Find the first occurrence of substring needle in string haystack. If the substring is not found, return -1.
Example 1
Input: haystack = "sadbutsad", needle = "sad"
Output: 0
Explanation: The substring "sad" starts at index 0.
Output: 0
Explanation: The substring "sad" starts at index 0.
Example 2
Input: haystack = "leetcode", needle = "leeto"
Output: -1
Explanation:
Output: -1
Explanation: