Implement the following operations of a queue using stacks.

• push(x) -- Push element x to the back of queue.
• pop() -- Removes the element from in front of queue.
• peek() -- Get the front element.
• empty() -- Return whether the queue is empty.

Notes:

• You must use only standard operations of a stack -- which means only push to top
  , peek/pop from top, size, and is empty operations are valid.
• Depending on your language, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

用棧來實現隊列，對比類似題目225. Implement Stack using Queues 是反過來用。

class Queue:
    # initialize your data structure here.
    def __init__(self):
        self.A, self.B = [], []

    # @param x, an integer
    # @return nothing
    def push(self, x):
        self.A.append(x)

    # @return an integer
    def pop(self):
        self.peek()
        return self.B.pop()
        
    # @return an integer
    def peek(self):
        if not self.B:
            while self.A:
                self.B.append(self.A.pop())
        return self.B[-1]
        
    # @return an boolean
    def empty(self):
        return not self.A and not self.B

C++:

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        stack<int> tmp;
        while (!s.empty()) {
            tmp.push(s.top());
            s.pop();
        }
        s.push(x);
        while (!tmp.empty()) {
            s.push(tmp.top());
            tmp.pop();
        }
    }

    // Removes the element from in front of queue.
    void pop(void) {
        s.pop();
    }

    // Get the front element.
    int peek(void) {
        return s.top();
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return s.empty();
    }

private:
    stack<int> s;
};

C++:

class Queue {
public:
    // Push element x to the back of queue.
    void push(int x) {
        _new.push(x);
    }
    
    void shiftStack() {
        if (_old.empty()) {
            while (!_new.empty()) {
                _old.push(_new.top());
                _new.pop();
            }
        }
    }

    // Removes the element from in front of queue.
    void pop(void) {
        shiftStack();
        if (!_old.empty()) _old.pop();
    }

    // Get the front element.
    int peek(void) {
        shiftStack();
        if (!_old.empty()) return _old.top();
        return 0;
    }

    // Return whether the queue is empty.
    bool empty(void) {
        return _old.empty() && _new.empty();
    }

private:
    stack<int> _old, _new;
};　　

類似題目：

[LeetCode] 225. Implement Stack using Queues 用隊列來實現棧

[LeetCode] 232. Implement Queue using Stacks 用棧來實現隊列