Second Largest Digit in a String

Given an alphanumeric string s, return the second largest numerical digit that appears in s, or -1 if it does not exist.

An alphanumeric string is a string consisting of lowercase English letters and digits.

Example 1:

Input: s = "dfa12321afd"
Output: 2
Explanation: The digits that appear in s are [1, 2, 3]. The second largest digit is 2.

Example 2:

Input: s = "abc1111"
Output: -1
Explanation: The digits that appear in s are [1]. There is no second largest digit. 

Approach

C++

#include <bits/stdc++.h>
using namespace std;

int secondHighest(string s)
{
    int n = s.size();
    int i = 0;
    set<int> st;
    while (i < n)
    {
        while (i < n && !(s[i] >= '0' && s[i] <= '9'))
            i++;
        while (i < n && s[i] >= '0' && s[i] <= '9')
        {
            st.insert(s[i] - '0');
            i++;
        }
    }
    if (st.size() >= 2)
    {
        vector<int> vec;
        for (auto it = st.begin(); it != st.end(); it++)
            vec.push_back(*it);
        sort(vec.begin(), vec.end());
        return vec[vec.size() - 2];
    }
    return -1;
}

int main()
{
    string s = "dfa12321afd";

    cout << secondHighest(s) << "\n";

    return 0;
}