问题描述

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).

用队列实现栈

代码

class MyStack {
public:
	queue<int> stack;
	/** Initialize your data structure here. */
    MyStack() {
        
    }
    
    /** Push element x onto stack. */
    void push(int x) {
        stack.push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
    	int len = stack.size();
		for(int i=0;i<len-1;i++) {
			int tmp = stack.front();
			stack.pop();
			stack.push(tmp);
		}
		int t = stack.front();
		stack.pop();
		return t;
    }
    
    /** Get the top element. */
    int top() {
        int len = stack.size();
		for(int i=0;i<len-1;i++) {
			int tmp = stack.front();
			stack.pop();
			stack.push(tmp);
		}
		int t = stack.front();
		stack.pop();
		stack.push(t);
		return t;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return stack.empty();
    }
};