[Leetcode] 3.Longest Substring Without Repeating Characters（unordered_map）

通过把未访问的结点放到unordered_map中来判断是否重复，代码如下：

class Solution {
public:
    int lengthOfLongestSubstring(string s) {
        if(s == "")
            return 0;
        unordered_map<char,int> hash;
        int len = 0;
        int res = 0;
        for(int i = 0;i < s.length();i ++)
        {
            if(hash.find(s[i]) != hash.end())
            {
                i = hash[s[i]];
                hash.clear();
                len = 0;
            }
            else
            {
                hash[s[i]] = i;
                len ++;
                res = max(len, res);
            }
        }
        return res;
    }
};