leetcode28 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
"""
"""
用切片的方法匹配字符
"""
class Solution:
    def strStr(self, haystack, needle):
        for i in range(len(haystack) - len(needle)+1):#bug此处不加1,input如果为两个空字符串,无法输出i
            if needle == haystack[i:i+len(needle)]:
                return i
        return -1