https://leetcode.com/problems/implement-strstr/submissions/

implement_strstr.py

class Solution:
    def strStr(self, haystack: str, needle: str) -> int:
        if needle == "":
            return 0
        if len(haystack) == 1 or len(haystack) == len(needle):
            return 0 if haystack == needle else -1

        if len(needle) > len(haystack):
            return -1

        h_pos = 0
        while h_pos < len(haystack):
            n_pos = 0
            while n_pos < len(needle) and h_pos < len(haystack) and haystack[h_pos] == needle[n_pos]:
                n_pos += 1
                h_pos += 1
            if n_pos == len(needle):
                return h_pos - n_pos
            h_pos = (h_pos - n_pos) + 1

        return -1