finds the index of a given substring if it exists, otherwise returns -1.

Instructions:

Implement 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() and Java's indexOf().

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.

Link to question: https://leetcode.com/problems/implement-strstr/