problem 3. Longest Substring Without Repeating Characters

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        vector<int> signs(256, -1);
        int max_length = 0;
        int last_valid = -1;
        for (size_t i = 0; i < s.size(); ++i){
            char c = s[i];
            int last_occ = signs[c];
            if (last_occ < last_valid){
                last_occ = last_valid;
            }
            if (last_occ > last_valid){
                last_valid = last_occ;
            }
            int length = i - last_occ;
            if (length > max_length){
                  max_length = length;
            }
            
            signs[c] = i;
        }
        if (max_length == 0){
            max_length = s.size();
        }
        return max_length;
    }
};