problem:3. 无重复字符的最长子串

思路：

Accode:

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int ans = 0;
        unordered_map<int,int> cnt;
        deque<int> deq;
        int len = s.length();
        for(int i=0;i<len;i++){
            while(deq.size() && cnt[s[i]]>=1){
                cnt[s[deq.front()]]--;
                if(cnt[s[deq.front()]]==0) cnt.erase(s[deq.front()]);  
                deq.pop_front();
            }
            deq.push_back(i);
            cnt[s[i]]++;
            ans = max(ans,(int)cnt.size());
        }
        return ans;
    }
};

时间复杂度：$o(n)$