3.Longest Substring Without Repeating Characters

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int res = 0, left = -1, n = s.size();
        unordered_map<int, int> m;
        for (int i = 0; i < n; ++i) {
            if (m.count(s[i]) && m[s[i]] > left) {
                left = m[s[i]];  
            }
            m[s[i]] = i;
            res = max(res, i - left);            
        }
        return res;
    }
};