无重复字符的最长子串

思路一：哈希表记录每个字符最后一次出现的位置

    int lengthOfLongestSubstring(string s) {
        if(s.empty())return 0;
        int n = s.length();
        vector<int> index(256, -1);
        int begin = -1, maxlen = 0;
        for(int i = 0;i < n;++i){
            begin = max(index[s[i]], begin);
            maxlen = max(maxlen, i - begin);
            index[s[i]] = i;
        }
        return maxlen;
    }

思路二：滑动窗口

    int lengthOfLongestSubstring(string s) {
        int left = 0, right = 0, len = s.length(), maxLen = 0;
        unordered_map<char, int> m;
        while(right < len){
            char c1 = s[right];
            ++m[c1];
            ++right;
            while(m[c1] > 1){
                char c2 = s[left];
                --m[c2];
                ++left;
            }
            maxLen = max(maxLen, right - left);
        }
        return maxLen;
    }