# 【LeetCode】 3. Longest Substring Without Repeating Characters

## Description
> Given a string, find the length of the longest substring without repeating characters.

> 給予一字串，請回傳最大子字串長度，其中沒有重複的字母。

## Example:

Example 1:

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

Example 2:

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

Example 3:

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

## Solution

* 用map記錄每個字元最後出現的位置。
* 記錄最後一次有重複字元的位置 = last。
* 現在位置-last是該位置目前的最大長度。

### Code

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        unordered_map<char,int> m;
        int ans = 0;
        int last = -1;
        for(int i = 0;i<(int)s.size();i++)
        {
            if(m.find(s[i])!=m.end())
            {
                last = last > m[s[i]] ? last : m[s[i]];
            }
            ans = ans > (i-last) ? ans : (i-last);
            m[s[i]]=i;
        }
        return ans;
    }
};

###### tags: `LeetCode` `C++`