LeetCode 225. Implement Stack using Queues

Question

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.

Example:

MyStack stack = new MyStack();

stack.push(1);
stack.push(2);  
stack.top();   // returns 2
stack.pop();   // returns 2
stack.empty(); // returns false

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).

Solution

• queue的先进先出策略与stack的后进先出策略正好相反。
• 使用double queues的方法模拟stack时，2个队列的空间不是必需的，也可以通过in-place的方法实现。
  • 在MyStack push的时候，将新加入的元素用循环移位的方法从back移动到front。具体方法是一个queue的pop-push循环。

  • 

• 在使用double queues的情况下有两种选择：
  1. 新加入的元素放于queue的front处，从front到back，元素从新到旧排列。这种情况push的时间代价为O(N)，pop的时间代价为O(1)。

     • 

  2. 新加入的元素放在queue的back处，从front到back，元素从旧到新排列。在push的时候需要通过一个pop-push循环获得最新的元素。这种情况push的时间代价O(1)，pop的时间代价为O(N)。

     • 

• C++ STL中的queue、stack的push()和pop()均没有返回值

In-place solution

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        q = new queue<int>;
    }
    
    ~MyStack() {delete q;}

    /** Push element x onto stack. */
    void push(int x) {
        q->push(x);
        int item = 0;
        for (int i=q->size()-1;i>0;i--) {
            item = q->front();
            q->pop();
            q->push(item);
        }
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        int item = q->front();
        q->pop();
        return item;
    }
    
    /** Get the top element. */
    int top() {
        return q->front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q->empty();
    }
private:
    queue<int> *q;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

2 queues solution1

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        q = new queue<int>;
    }
    
    ~MyStack() {delete q;}

    /** Push element x onto stack. */
    void push(int x) {
        queue<int> *tmp = new queue<int>;
        int item =0;
        tmp->push(x);
        while(!q->empty()) {
            item = q->front();
            q->pop();
            tmp->push(item);
        }
        delete q;
        q = tmp;
    }
    
    /** Removes the element on top of the stack and returns that that element. */
    int pop() {
        int item = q->front();
        q->pop();
        return item;
    }
    
    /** Get the top element. */
    int top() {
        return q->front();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q->empty();
    }
private:
    queue<int> *q;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */

2 queues solution2

class MyStack {
public:
    /** Initialize your data structure here. */
    MyStack() {
        q = new queue<int>;
    }
    
    ~MyStack() {delete q;}

    /** Push element x onto stack. */
    void push(int x) {
        q->push(x);
    }
    
    /** Removes the element on top of the stack and returns that element. */
    int pop() {
        queue<int> *tmp = new queue<int>;
        int item =0;
        while(q->size()>1) {
            item = q->front();
            q->pop();
            tmp->push(item);
        }
        item = q->front();
        delete q;
        q = tmp;
        return item;
    }
    
    /** Get the top element. */
    int top() {
        return q->back();
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return q->empty();
    }
private:
    queue<int> *q;
};

/**
 * Your MyStack object will be instantiated and called as such:
 * MyStack* obj = new MyStack();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->top();
 * bool param_4 = obj->empty();
 */