[1,3],3
["Push","Push","Pop","Push"]
读取 1 并自动推入数组 -> [1] 读取 2 并自动推入数组,然后删除它 -> [1] 读取 3 并自动推入数组 -> [1,3]
[1,2,3], 3
["Push","Push","Push"]
[1,2],4
["Push","Push"]
只需要读取前 2 个数字就可以停止。
[2,3,4],4
["Push","Pop","Push","Push","Push"]