28. Find the Index of the First Occurrence in a String - leetcode - python - easy

 

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.

class Solution:

    def strStr(self, haystack: str, needle: str) -> int:

        if not needle:

            return 0

        for i in range(len(haystack) - len(needle) + 1):

            if haystack[i:i+len(needle)] == needle:

                return i

        return -1

Explanation (Simple Version)

If needle is empty → return 0 (by definition).

Loop through haystack where needle can fit: Check if the slice haystack[i:i+len(needle)] matches needle. If yes → return the starting index i.

If no match → return -1.