strStr

class Solution {
    /**
     * Returns a index to the first occurrence of target in source,
     * or -1  if target is not part of source.
     * @param source string to be scanned.
     * @param target string containing the sequence of characters to match.
     */
    public int strStr(String source, String target) {
        // write your code here
        if (source == null && target == null){
            return -1;
        }
        if (source == null){
            return -1;
        }
        if (target == null){
            return -1;
        }
        for (int i = 0; i < source.length() - target.length() + 1; i++){
            int j = 0;
            for ( ; j < target.length(); j++){
                if (source.charAt(i + j) != target.charAt(j)){
                    break;
                }
            }
            if (j == target.length()){
                    return i;
                }
        }
        return -1;
    }
}