28. 实现 strStr()

class Solution {
    public int strStr(String haystack, String needle) {
        if (needle.length() == 0) {
            return 0;
        }
        for (int i = 0; i + needle.length() - 1 < haystack.length(); i++) {
            if (needle.equals(haystack.substring(i, i + needle.length()))) {
                return i;
            }
        }
        return -1;
    }
}