232. Implement Queue using Stacks

Since Queue is FIFO but Stack is FILO. If we need to use Stack to implement a Queue, we need to use at least two Stacks. So we use one stack which only handle Push operations, and another Stack which only handle Pop/Peek operations. And we move elements from the Pop only Stack to the other one when Pop/Peek get called. It will reverse the FILO stack elements sequence after that. So we get a FIFO sequence.

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type Stack []int

func (s *Stack)Empty() bool {
    return len(*s) == 0
}

func (s *Stack)Push(x int) {
    *s = append(*s, x)
}

func (s *Stack)Pop() int {
    var ret int
    if !s.Empty() {
        lastIndex := len(*s) - 1
        ret = (*s)[lastIndex]
        *s = (*s)[:lastIndex]
    }
    return ret
}

func (s *Stack)Peek() int {
    var ret int
    if !s.Empty() {
        ret = (*s)[len(*s) - 1]
    }
    return ret
}

type MyQueue struct {
    m, n *Stack
}

func Constructor() MyQueue {
    return MyQueue{
        &Stack{},
        &Stack{},
    }
}

func (q *MyQueue)Push(x int) {
    q.m.Push(x)
}

func (q *MyQueue)Pop() int {
    q.Peek()
    return q.n.Pop()
}

func (q *MyQueue)Peek() int {
    if q.n.Empty() {
        for !q.m.Empty() {
            q.n.Push(q.m.Pop())
        }
    }
    return q.n.Peek()
}


func (q *MyQueue)Empty() bool {
    return q.m.Empty() && q.n.Empty()
}