Second Largest Digit in a String

给一个string, 找第二大的数字.

class Solution {
    public int secondHighest(String s) {
        Set<Integer> set = new HashSet<>();
        for(char c : s.toCharArray()){
            if('0' <= c && c <= '9'){
                set.add(c - '0');
            }
        }
        boolean largest = false;
        for(int i = 9; i >= 0; i--){
            if(set.contains(i)){
                if(!largest)
                    largest = true;
                else{
                    return i;
                }
            }
        }
        return -1;
    }
}