Implement strStr()

 1 public int strStr(String haystack, String needle) {
 2     if(haystack == null) {
 3         return -1;
 4     }
 5     
 6     if(needle == null) {
 7         return haystack.length();
 8     }
 9     
10     if(haystack.length() < needle.length()) {
11         return -1;
12     }
13     
14     for(int i = 0; i <= haystack.length() - needle.length(); i++) {
15         String tmp = haystack.substring(i, i+needle.length());
16         if(tmp.equals(needle)) {
17             return i;
18         }
19     }
20     
21     return -1;
22 }