28. Implement strStr()

public int strStr(String haystack, String needle) {
if(needle == null || needle.length() == 0) {
return 0;
}
if(haystack.length() < needle.length()) {
return -1;
}
for(int i = 0; i <= haystack.length()-needle.length(); i++) {//需要<=
int j = 0;
while(j < needle.length()) {
if(haystack.charAt(i+j) == needle.charAt(j)) {
j++;
} else {
break;
}
if(j == needle.length()) {
return i;
}
}
}
return -1;
}