Smallest window in a string containing all the characters of another string

Given two strings S and P. Find the smallest window in the string S consisting of all the characters(including duplicates) of the string P. Return “-1” in case there is no such window present. In case there are multiple such windows of same length, return the one with the least starting index.

• Time : O(n)
• Space : O(n)

class Solution {
public:
    //Function to find the smallest window in the string s consisting
    //of all the characters of string p.
    string smallestWindow(string s, string p) {
        unordered_map<char, int> counter;
        unordered_map<char, int> sCounter;
        for(auto& ch : p)
            counter[ch]++;
        int freq = counter.size(), res = INT_MAX, pos;
        int l = 0, r = 0, n = s.length();
        while(r < n) {
            while(r < n and freq) {
                if(counter.count(s[r])) {
                    if(counter[s[r]] == ++sCounter[s[r]]) freq--;
                }
                r++;
            }
            while(l < r and !freq) {
                if(res > r - l) {
                    res = r - l; pos = l;
                }
                if(counter.count(s[l])) {
                    if(counter[s[l]] == sCounter[s[l]]--) freq++;
                }
                l++;
            }
        }

        return res == INT_MAX ? "-1" : s.substr(pos, res);
    }
};