Leetcode-3 longest string without repeated characters (sliding window, unordered_set, st.find (string[i])

（1） summary ： Master the idea of sliding window （ Head pointer is used in this problem while）,unordered_set, set.find(element) How to use .

（2）map and set It's realized by red black tree , Medium find The complexity of operation time is O(logN).

（3）STL There is unordered_map and unordered_set Two unordered containers . When using, add the corresponding header file .