3. Longest Substring Without Repeating Characters

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

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.

---

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        if(s.empty()){
            return 0;
        }
        int max=1;
        unordered_map<char,int> pos;
        int start=0;
        
        for(int i=0;i<s.size();i++){
            if(pos.find(s[i])!=pos.end()){
                
                int tmp = pos.size();
                max=tmp>max?tmp:max;
                int newstart=pos[s[i]]+1;
                for(int j=start;j<=pos[s[i]];j++){
                    pos.erase(s[j]);
                }
                start=newstart;
            }
            
            pos[s[i]]=i;
        }
        
        int tmp = pos.size();
        max=tmp>max?tmp:max;
        return max;
    }
};

grandyang解法一：

滑动窗口问题，用两个指针分别代表窗口的两端，保证窗口里的字母都不相同。

hash table用来存字母上一次出现的index。

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        int res = 0, left = -1, n = s.size();
        unordered_map<int, int> m;
        for (int i = 0; i < n; ++i) {
            if (m.count(s[i]) && m[s[i]] > left) {
                left = m[s[i]];  
            }
            m[s[i]] = i;
            res = max(res, i - left);            
        }
        return res;
    }
};

grandyang解法二：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        vector<int> m(128, -1);
        int res = 0, left = -1;
        for (int i = 0; i < s.size(); ++i) {
            left = max(left, m[s[i]]);
            m[s[i]] = i;
            res = max(res, i - left);
        }
        return res;
    }
};