28. Implement strStr()

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:

1 2	Input: haystack = "hello", needle = "ll" Output: 2

Example 2:

1 2	Input: haystack = "aaaaa", needle = "bba" Output: -1

Example 3:

1 2	Input: haystack = "", needle = "" Output: 0

Constraints:

1
2
3

0 <= haystack.length, needle.length <= 5 * 10ˆ4

haystack and needle consist of only lower-case English characters.

回傳子字串index，可以用find()

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