public class Solution
extends Object
28 - Implement strStr()\.
Easy
Implement [strStr()](http://www.cplusplus.com/reference/cstring/strstr/).
Return the index of the first occurrence of needle in haystack, or `-1` if `needle` is not part of `haystack`.
**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()](http://www.cplusplus.com/reference/cstring/strstr/) and Java's [indexOf()](https://docs.oracle.com/javase/7/docs/api/java/lang/String.html#indexOf(java.lang.String)).
**Example 1:**
**Input:** haystack = "hello", needle = "ll"
**Output:** 2
**Example 2:**
**Input:** haystack = "aaaaa", needle = "bba"
**Output:** -1
**Example 3:**
**Input:** haystack = "", needle = ""
**Output:** 0
**Constraints:**
* 0 <= haystack.length, needle.length <= 5 * 104
* `haystack` and `needle` consist of only lower-case English characters.