其实不用队列，直接用数据存放增加进来并且不重复的字符就好了，后面遍历一下数组就得出结果了 class Solution { public: //Insert one char from stringstream unordered_map<char> mp; vector<char> buf; void Insert(char ch) { if (mp.find(ch) == mp.end()) { buf.push_back(ch); } mp[ch]++; } //return the first appearence once char in current stringstream char FirstAppearingOnce() { for (int i = 0; i < buf.size(); i++) { if (mp[buf[i]] == 1) { return buf[i]; } } return '#'; } };</char></char>