class Solution:
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        if needle == "" or needle == haystack:
            return 0
        elif len(needle)> len(haystack):
            return -1
        else:
            for i in range(len(haystack)-1):
                if haystack[i] == needle[0]:
                    n = len(needle)
                    #print(haystack[i:i+n]) 
                    if haystack[i:i+n] == needle:
                        return i
                    else:
                        pass
            return -1

思路：遇到与needle首字符相同，则比较是否有包含。

注意：空字符串，needle更长，单字符的特殊情况，注意字符串切片用法

拓展：KMP算法