First Occurrence of Substring | Dataford Interview Questions

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Problem

TomTom navigation software may need to locate the first occurrence of a search phrase within a route instruction or map label. Given two strings, haystack and needle, return the index of the first character where needle occurs in haystack.

If needle does not occur, return -1. If needle is empty, return 0.

Formal Specification

Implement str_str(haystack, needle):

Input: Two strings, haystack and needle.
Output: An integer representing the starting index of the first occurrence of needle in haystack, or -1 if no occurrence exists.
Matching is case-sensitive, and characters must match contiguously.

For the target complexity, preprocess needle using the Knuth-Morris-Pratt algorithm rather than restarting comparisons after every mismatch.

Constraints

0 <= len(haystack) <= 10^5
0 <= len(needle) <= 10^4
Both strings contain printable ASCII characters
Matching is case-sensitive
The target complexity is O(len(haystack) + len(needle)) time

Function Signature

def str_str(haystack, needle):