滑动窗口提高《无重复字符的最长子串》时间效率

class Solution {

public:

    int lengthOfLongestSubstring(string s) {

        unordered_map<char,int> m;        

        int sum=0;

        int left=0,i=0;

        while(i<s.size()){        

                m[s[i]]++;

                while(m[s[i]]>1){

                    m[s[left]]--;

                    left++;

                }

                i++;

            sum=max(sum,i-left);

        }

        return sum;

    }

};