// attention: pay attention the the end index limit on all arrays (including strings)

class Solution {
    public int strStr(String haystack, String needle) {
        if (null == haystack) return -1;
        if (null == needle) return -1;
        if (needle.isEmpty()) return 0; // if empty string, return 0

        for (int i = 0; i <= haystack.length() - needle.length(); i++) {
            boolean isFound = true;
            for (int j = 0; j < needle.length(); j++) {
                if (haystack.charAt(i+j) != needle.charAt(j)) {
                    isFound = false;
                    break;
                }
            }

            if (isFound) {
                return i;
            }
        }
        return -1;
    }
};