28. Implement strStr()

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

Example 1:

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

Example 2:

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

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().

Constraints:

• haystack and needle consist only of lowercase English characters.

# @param {String} haystack
# @param {String} needle
# @return {Integer}
def str_str(haystack, needle)
  if !haystack.empty? && needle.empty?
    return 0
  end
  
  if haystack.include?(needle)
    return haystack.index(needle)
  end
  
  return -1
  
end