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# 3. Longest Substring Without Repeating Characters

## 1.問題

* 給予一個string, 找出最長的沒有重複字元的substring

![](/files/-LJDIcBYkbN81ovtqFDe)

## 2.想法

* 用vector來記錄曾經碰到過的字元, 一旦碰到第二次則將從start開始的紀錄取消並且寫入新的紀錄, 移動start, 紀錄長度為start到當前位置的長度
* 用vector代替map存放字母出現次數的好處是: 將會依照字母順序
* 對字串中每個子母進行標記: 出現兩次則清空, 並將first移到第一個重複字的下一個字母

## 3.程式碼

```
class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        vector<int> cnt(128, 0);
        int first = 0, size = s.size(), maxSize = 0;
        for (int i = 0; i < size; i++) {
            cnt[s[i]]++;
            if (cnt[s[i]] > 1) {
                //重要: 將first移動到第一個重複字出現的下一個字母
                for (; s[first] != s[i]; first++) {
                    cnt[s[first]] = 0;
                }
                first++;
            }
            maxSize = max(i - first + 1, maxSize); 
        } 
        return maxSize;
    }
};
```

## 4.Performance

![](/files/-LJDItV1yWwdnNcMdC6J)


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