leetcode-225-Implement Stack using Queues

                       Implement Stack using Queues

  

Implement the following operations of a stack using queues.

• push(x) -- Push element x onto stack.
• pop() -- Removes the element on top of the stack.
• top() -- Get the top element.
• empty() -- Return whether the stack is empty.

Notes:

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

Update (2015-06-11):
The class name of the Java function had been updated to MyStack instead of Stack.

用队列实现栈。

用两个队列即可实现栈。

class Stack {
public:
    queueq1,q2;
    // Push element x onto stack.
    void push(int x) {
        q1.push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        while (!q1.empty()) {
            if (q1.size() > 1)
                    q2.push(q1.front());
            q1.pop();
        }
        while (!q2.empty()) {
            q1.push(q2.front());
            q2.pop();
        }
    }

    // Get the top element.
    int top() {
        return q1.back();
    }

    // Return whether the stack is empty.
    bool empty() {
        return q1.empty();
    }
};