My solution is very similar to yours, but most of the test cases have failed. can you tell me why?

    public static int numberNeeded(String first, String second) {

    int numberNeeded = 0;
    Map<Character, Integer> count = new HashMap<>();
    for(char c : first.toCharArray()){
        if(count.containsKey(c))
            count.put(c, count.get(c)+1);
        else
            count.put(c, 1);
    }

    for(char c : second.toCharArray()){
        if(count.containsKey(c))
            count.put(c, count.get(c)-1);
        else
            count.put(c, 1);
    }

    for(int value: count.values()){
        if(value != 0){
            numberNeeded++;
        }
    }

    return numberNeeded;
}