28 Implement strstr()

题目不难，但是需要注意的地方很多。首先判断如果两个字符串长度不满足关系，可以直接返回。 不然进行双重循环，每次循环首先判断长度，然后进行匹配，如果都匹配不了，返回-1

class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        target_length = len(needle)
        pattern_length = len(haystack)
        if pattern_length < target_length:
            return -1
        if target_length == 0:
            return 0
        for i in range(pattern_length):
            start_position, end_position = i, i + target_length
            if end_position > pattern_length:
                return -1
            f = True
            for j in range(start_position, end_position):
                if haystack[j] != needle[j-start_position]:
                    f = False
                    break
            if f is True:
                return i
        return -1Copy
class Solution(object):
    def strStr(self, haystack, needle):
        """
        :type haystack: str
        :type needle: str
        :rtype: int
        """
        for i in range(len(haystack) + 1):
            for j in range(len(needle) + 1):
                if j == len(needle):
                    return i
                if i + j == len(haystack):
                    return -1
                if needle[j]!=haystack[i + j]:
                    break