Longest K unique characters substring

Given a string you need to print the size of the longest possible substring that has exactly K unique characters. If there is no possible substring then print -1.

• Time : O(n)
• Space : O(1)
  • require constant space

class Solution {
public:
    int longestKSubstr(string s, int k) {
        unordered_map<char, int> counter;
        int res = -1, l = 0, r = 0, n = s.length();
        while(r < n) {
            if(++counter[s[r++]] == 1) k--;
            if(k == 0)
                res = max(res, r - l);
            else {
                while(l < r and k < 0) {
                    if(--counter[s[l++]] == 0) k++;
                }
            }
        }
        return res;
    }
};