Leetcode 两个队列实现栈 swift

class MyStack {
    var queue1: [Int] = []
    var queue2: [Int] = []
    init() {

    }
    
    func push(_ x: Int) {
        queue2.append(x)
        while !queue1.isEmpty {
            queue2.append(queue1.removeFirst())
        }
        swap(&queue1, &queue2)
    }
    
    func pop() -> Int {
        let r: Int = queue1.removeFirst()
        return r
    }
    
    func top() -> Int {
        var r: Int = 0
        if !queue1.isEmpty {
            r = queue1.first!
        }
        return r
    }
    
    func empty() -> Bool {
        return queue1.isEmpty
    }
}

/**
 * Your MyStack object will be instantiated and called as such:
 * let obj = MyStack()
 * obj.push(x)
 * let ret_2: Int = obj.pop()
 * let ret_3: Int = obj.top()
 * let ret_4: Bool = obj.empty()
 */