Implement strStr().
Returns a pointer to the first occurrence of needle in haystack, or null if needle is not part of haystack.
暴力算法的复杂度是 O(m*n)，代码如下。更高效的的算法有KMP算法、Boyer-Mooer算法和Rabin-Karp算法。面试中暴力算法足够了，一定要写得没有BUG。
{% codesnippet "./code/strstr-1."+book.suffix, language=book.suffix %}{% endcodesnippet %}
{% codesnippet "./code/strstr-2."+book.suffix, language=book.suffix %}{% endcodesnippet %}