LintCode 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(target.length() == 0) return 0;
        if(source.length() == 0) return -1;
        
        for(int i = 0; i < source.length(); i++){
            boolean flag = true;
            for( int j = 0; j < target.length(); j++){
                if(source.charAt(i + j) == target.charAt(j)){
                    
                }
                else{
                    flag = false;
                    break;
                }
            }
            
            if(flag) return i;            
        }
        
        return -1;
    }
}