连接两个字符串中的不同字符

public class Solution {
    /*
     * @param : the 1st string
     * @param : the 2nd string
     * @return: uncommon characters of given strings
     */
    public String concatenetedString(String s1, String s2) {
        StringBuffer sb = new StringBuffer();
        Map<Character, Integer> map = new HashMap<Character, Integer>();
        for (char c : s2.toCharArray()) {
            map.put(c, 1);
        }
        
        for (char c : s1.toCharArray()) {
            if (!map.containsKey(c)) {
                sb.append(c);
            } else {
                map.put(c, 2);
            }
        }
        
        for (char c : s2.toCharArray()) {
            if (map.get(c) != 2) {
                sb.append(c);
            }
        }
        return sb.toString();
    }
}