1796.字符串中第二大的数字

class Solution {
public:
    int secondHighest(string s) {
        int first = -1;
        int second = -1;
        for (int i = 0; i < s.size(); i++) {
            if (s[i] <= '9') {
                if (s[i] - '0' > first) {
                    second = first;
                    first = s[i] - '0';
                } else if (s[i] - '0' < first && s[i] - '0' > second)
                    second = s[i] - '0';
            }
        }
        return second;
    }
};