implement-strstr

Implement strStr().

Return the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Example 1:

Input: haystack = "hello", needle = "ll"
Output: 2
Example 2:

Input: haystack = "aaaaa", needle = "bba"
Output: -1
Clarification:

What should we return when needle is an empty string? This is a great question to ask during an interview.

For the purpose of this problem, we will return 0 when needle is an empty string. This is consistent to C's strstr() and Java's indexOf().

My code:
class Solution:
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if needle=="":
            return 0
        length=len(needle)
        length2=len(haystack)
        if length2==length:
            if haystack==needle:
                return 0
            else:
                return -1
        for i in range(length2-length+1):
            if haystack[i:i+length]==needle:
                return i
        return -1
Solution:
https://leetcode.com/problems/implement-strstr/