Find the index of the first occurrence in a string

class Solution:
    def getNext(self, patt):
        next = [0] * len(patt)
        j = 0
        for i in range(1, len(patt)):
            while j > 0 and patt[i] != patt[j]:
                j = next[j - 1]
            if patt[i] == patt[j]:
                j += 1
            next[i] = j
        return next

    def strStr(self, haystack: str, needle: str) -> int:
        next = self.getNext(needle)
        j = 0
        for i in range(len(haystack)):
            while j > 0 and haystack[i] != needle[j]:
                j = next[j - 1]
            if haystack[i] == needle[j]:
                j += 1
            if j == len(needle):
                return i - j + 1
        return -1

Find the Index of the First Occurrence in a String

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.

Example 1:

Input: haystack = "sadbutsad", needle = "sad"
Output: 0
Explanation: "sad" occurs at index 0 and 6.
The first occurrence is at index 0, so we return 0.

Example 2:

Input: haystack = "leetcode", needle = "leeto"
Output: -1
Explanation: "leeto" did not occur in "leetcode", so we return -1.

Constraints:

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