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# Q3. Longest Substring Without Repeating Characters

直达：<https://leetcode.com/problems/longest-substring-without-repeating-characters/description/>

Given a string, find the length of the**longest substring**without repeating characters.

**Examples:**

Given`"abcabcbb"`, the answer is`"abc"`, which the length is 3.

Given`"bbbbb"`, the answer is`"b"`, with the length of 1.

Given`"pwwkew"`, the answer is`"wke"`, with the length of 3. Note that the answer must be a**substring**,`"pwke"`is asubsequenceand not a substring.

## 分析

用哈希表存储每个元素的下标（`hash[s[i]] = i`）如重复，可用新的下标覆盖旧的下标。遍历字符串，如果哈希表中不存在该字符，

## C++代码

```cpp
class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        unordered_map <char, int> hash;
        int res = 0;
        int cur_res = 0;
        for (int i = 0; i<s.size(); i++){
            if (hash.find(s[i]) == hash.end()){
                cur_res += 1;
            }else{
                cur_res = min((i-hash[s[i]]), (cur_res+1));
            }
            hash[s[i]] = i;
            res = res>cur_res?res:cur_res;
        }
        return res;
    }
};
```