Index of First Occurennce in String

Here is the link to the problem: Index of First Occurrence in String

Problem Statement:

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.

Examples:

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.

My Solution

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        l = 0
        r = len(needle)
        length = len(haystack)
        while r <= length:
            if needle == haystack[l:r]:
                return l
            l += 1
            r += 1

My logic is simple, l would be 0 and r would be whatever the length of the needle. We check if needle matches the slice of word from the haystack if it does we return 1 else -1. We increase l and r accordingly. Also, this condition should continue until the length of r is less than or equal to length of the haystack.

Some other solutions I found in leetcode:

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if needle in haystack:
            return haystack.index(needle)
        return -1

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        for i in range(0, len(haystack)-len(needle)+1):
            if haystack[i:i+len(needle)] == needle:
                return i
        return -1

Also, interesting comment that I found in the description.

""" Who marked this as Easy? It’s not easy at all. Though you can easily produce O(mn) solution, it is not accepted for python. You need KMP algorithm to produce O(m + n) solution! """