leetcode解题: Implement Queue using Stacks (232)

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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).

解法1：Two Queue

stack和queue的区别是前者是FILO, 后者是FIFO， 如果用另外一个stack去反倒回来就得到了queue一样的效果。
实现起来，用一个buffer stack存储应该的顺序，peek和pop都应该从buffer中取，如果读取buffer的时候为空的话，则需要把数据从原stack移到buffer。
C++

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class Queue {
private:
    stack<int> mstack;
    stack<int> buffer;
public:
    // Push element x to the back of queue.
    void push(int x) {
        mstack.push(x);
    }
    // Removes the element from in front of queue.
    void pop(void) {
        if (buffer.empty()) {
            while (!mstack.empty()) {
                buffer.push(mstack.top());
                mstack.pop();
            }
        }
        buffer.pop();
    }
    // Get the front element.
    int peek(void) {
        if (buffer.empty()) {
             while (!mstack.empty()) {
                buffer.push(mstack.top());
                mstack.pop();
            }
        }
        return buffer.top();
    }
    // Return whether the queue is empty.
    bool empty(void) {
        return mstack.empty() && buffer.empty();
    }
};

Java
只有在push的时候，如果当前的stack里面存在元素的话，先把元素存在另外一个stack里面。然后把要push的元素push了之后再把buffer里面的元素放回stack。

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class MyQueue {
    /** Initialize your data structure here. */
    Stack<Integer> stack;
    public MyQueue() {
        stack = new Stack<Integer>();
    }
    /** Push element x to the back of queue. */
    public void push(int x) {
        Stack<Integer> temp = new Stack<Integer>();
        while (!stack.isEmpty()) {
            temp.push(stack.pop());
        }
        stack.push(x);
        while (!temp.isEmpty()) {
            stack.push(temp.pop());
        }
    }
    /** Removes the element from in front of queue and returns that element. */
    public int pop() {
        return stack.pop();
    }
    /** Get the front element. */
    public int peek() {
        return stack.peek();
    }
    /** Returns whether the queue is empty. */
    public boolean empty() {
        return stack.isEmpty();
    }
}
/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */