225. Implement Stack using Queues

Easy

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.

• int pop() Removes the element on the top of the stack and returns it.

• int top() Returns the element on the top of the stack.

• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.

• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue's standard operations.

Example 1:

Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]
Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False

Constraints:

• 1 <= x <= 9

• At most 100 calls will be made to push, pop, top, and empty.

• All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

解題

type MyStack struct {
    queue1 []int
    queue2 []int
}


func Constructor() MyStack {
    return MyStack{}
}


func (this *MyStack) Push(x int)  {
    this.queue1 = append(this.queue1, x)
}


func (this *MyStack) Pop() int {
    res := -1

    for len(this.queue1) > 1 {
        this.queue2 = append(this.queue2, this.queue1[0])
        this.queue1 = this.queue1[1:]
    }

    res = this.queue1[0]

    this.queue1 = this.queue2
    this.queue2 = []int{}

    return res
}

func (this *MyStack) Top() int {
    res := -1

    for len(this.queue1) > 0 {
        if len(this.queue1) == 1 {
            res = this.queue1[0]
        }
        this.queue2 = append(this.queue2, this.queue1[0])
        this.queue1 = this.queue1[1:]
    }

    this.queue1 = this.queue2
    this.queue2 = []int{}

    return res
}


func (this *MyStack) Empty() bool {
    return len(this.queue1) == 0 && len(this.queue2) == 0
}


/**
 * Your MyStack object will be instantiated and called as such:
 * obj := Constructor();
 * obj.Push(x);
 * param_2 := obj.Pop();
 * param_3 := obj.Top();
 * param_4 := obj.Empty();
 */