class Solution {

public:

    int lengthOfLongestSubstring(string s) {

    	unordered_map<char,int> hashmap;

    	int left = 0,max_len=0;

    	for (int i=0; i < s.size(); i++){

    		if (hashmap.find(s[i]) != hashmap.end()){

    			left = max(left, hashmap[s[i]] +1);  // 更新左边界

    		}

    		max_len = max(max_len,i-left+1)

    		hashmap[s[i]] = i;  // 遍历的同时hashmap 加入字符对应位置

    	}

    	return max_len;

    }};