Length of the longest substring

Given a string S, find the length of the longest substring without repeating characters.

• Time : O(n)
• Space : O(n)

class Solution{
public:
    int longestUniqueSubsttr(string S){
        int dup = 0, l = 0, r = 0, res = 0, n = S.length();
        unordered_map<char, int> freq;
        
        while(r < n) {
            while(!dup and r < n) {
                if(++freq[S[r++]] == 2) dup++;
                if(!dup) res = max(res, r - l);
            }
            while(dup and l < r) {
                if(--freq[S[l++]] == 1) dup--;
            }
            
        }
        
        return res;
    }
};