1.Use two pointers: start and end to represent a window.
2.Move end to find a valid window.
3.When a valid window is found, move start to find a smaller window.
4 需要counter存储T中各个字母出现的次数
1.Use two pointers: start and end to represent a window.
2.Move end to find a valid window.
3.When a valid window is found, move start to find a smaller window.
4 需要counter存储T中各个字母出现的次数