Implement strStr().
Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.
就是找到子串第一次出现的 位置。
public int strStr(String haystack, String needle) {
if (haystack == null || needle == null) {
return -1;
}
for (int i = 0; i < haystack.length() - needle.length() +