---
tags: LeetCode,Top 100 Liked Questions
---
# 3.Longest Substring Without Repeating Characters
https://leetcode.com/problems/longest-substring-without-repeating-characters/
## 題目敘述
Given a string s, find the length of the longest substring without repeating characters.
找出**不重複**的**最長連續**子字串

Example 1:

Input: s = "abcabcbb"
Output: 3
Explanation: The answer is "abc", with the length of 3.

Example 2:

Input: s = "bbbbb"
Output: 1
Explanation: The answer is "b", with the length of 1.

Example 3:

Input: s = "pwwkew"
Output: 3
Explanation: The answer is "wke", with the length of 3.
Notice that the answer must be a substring, "pwke" is a subsequence and not a substring.

## 解題想法
我們可以一個個char遍歷，當右邊的char出現在遍歷過的char中，去掉重複的char，將新的char加入遍歷過的char，重覆到字串結尾
//利用HashMap紀錄位置和出現過的char
- 1.silde window
silde window往字串右邊滑動，右邊char有3種情況:
- 1.char出現過且在window中->window的left指向此char的前一格
- 2.char出現過不在window中(不影響window)->window直接往右
- 3.char沒出現過->window直接往右

public:
    int lengthOfLongestSubstring(string s) {
        int res=0,left=-1;//res:紀錄最長子字串長度 left:紀錄silde window 的左邊界
        unordered_map<int, int> m;
        for(int i=0;i<s.length();i++){
            if(m.count(s[i]) && m[s[i]]>left){
            //m.count(s[i])-->有沒有出現過 
            //m[s[i]]>left -->char有沒有在window中
                left=m[s[i]];//若char出現且在window->把left指向現在位置
            }
            m[s[i]]=i;
            res=max(res,i-left);
        }
        return res;
    }
};

時間複雜度 O(N)