Implement strStr().txt

problem：
Implement strStr().

Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.
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solution：
//考察字符串数组与指针的使用，直接暴力搜索，过小集合，大集合超时
char *strStr(char *haystack, char *needle)
{
  if(needle == NULL || *needle == '\0')
		return haystack;
	char *p = haystack;
	while(*p != '\0')
	{
		char *pBegin = p;
		char *q = needle;
		while(*q != '\0'&&*p != '\0' && *p == *q)
		{
				p++;
				q++;
		}
		if(*q == '\0')
			return pBegin;
		p = pBegin + 1;
	}
	return NULL;
}


///////improved
//上面的暴力搜索外层循环了n次，n为haystack的长度，实际上只需要循环n-m+1次，因为长度不够m的时候肯定不可能匹配
//设个指针tail，开始时tail与开始间隔m，当tail指到字符串结束出，循环停止
char *strStr(char *haystack, char *needle)
{
  if(needle == NULL || *needle == '\0')
		return haystack;
	char *p = haystack;
	char *q = needle;
	char *tail = haystack;
	while(*++q != '\0')
		tail++;
	while(*tail != '\0')
	{
		char *pBegin = p;
		q = needle;	
		while(*q != '\0'&&*p != '\0' && *p == *q)
		{
			p++;
			q++;
		}
		if(*q == '\0')
			return pBegin;
		p = pBegin + 1;
		tail++;
	}
	return NULL;
}