Implement strStr()

Implement strStr().

Given two strings needle and haystack, 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

Constraints:

• 1 <= haystack.length, needle.length <= 104
• haystack and needle consist of only lowercase English characters.

SOLUTION:

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        n = len(haystack)
        k = len(needle)
        exists = {needle}
        for i in range(0, n - k + 1):
            if haystack[i : i + k] in exists:
                return i
        return -1