Implement strStr()

Implement strStr().

Returns the index of the first occurrence of needle in haystack, or -1 if needle is not part of haystack.

Navie solution: O(m*n), start from the first char in haystack try to match needle int its substring.

public int strStr(String haystack, String needle) {
        if (haystack==null||needle==null||haystack.length()<needle.length()) {
            return -1;
        }

        if (needle.isEmpty()) {
            return 0;
        }

        int difference = haystack.length() - needle.length() + 1;
        for (int i=0; i<difference; i++) {
            int start = i;
            for (int j=0; j<needle.length(); j++) {
                if (haystack.charAt(start)==needle.charAt(j)) {
                    if (j==needle.length()-1) {
                        return i;
                    }
                    start = start + 1;
                } else {
                    break;
                }
            }
        }

        return -1;
    }