、有一种栈，这种栈是按数组索引减小的方向增长的。初始时栈为空且top=21。往栈中压入3个字符后，索引top=18，栈顶元素为list[top]=C。-, There is a stack, the stack is reduced by the array index of the direction of growth. The initial stack is empty and the top = 21. Pressure to the stack into the three char