Naivemethod: Start checking once
haystack[i] === needle.
index of the
first occurrence of needle in haystack, or -1 if needle is not part of haystack.
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().
Input: haystack = "hello", needle = "ll"