Implementing Stacks 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.

Analyse: Using two queues to store the information. q is used to mock the stack while help is used to store temporary values.

1. Time Exceeded Limit Version

　 It's because transfering temporary queue to the original queue costs a lot of time. 

class Stack {
private:
    queue<int> q;
    queue<int> help;
public:
    // Push element x onto stack.
    void push(int x) {
        q.push(x);
    }

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

    // Get the top element.
    int top() {
        while(q.size() > 1){
            help.push(q.front());
            q.pop();
        }
        int result = q.front();
        while(!help.empty()){
            q.push(help.front());
        }
    }

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

2. Set a 2-size array to respectively represent the two queue. When it comes to copy the temporary queue to the original queue, just switch the index.

    Runtime: 0ms.

class Stack {
private:
    queue<int> q[2];
    int index = 0;
public:
    // Push element x onto stack.
    void push(int x) {
        q[index].push(x);
    }

    // Removes the element on top of the stack.
    void pop() {
        while(q[index].size() > 1){ //transfer all other elements into the helper queue
            q[1 - index].push(q[index].front());
            q[index].pop();
        }
        q[index].pop();
        index = 1 - index;
    }

    // Get the top element.
    int top() {
        while(q[index].size() > 1){ //transfer all other elements into the helper queue
            q[1 - index].push(q[index].front());
            q[index].pop();
        }
        int result = q[index].front();
        q[1- index].push(result);
        q[index].pop();
        index = 1 - index;
        return result;
    }

    // Return whether the stack is empty.
    bool empty() {
        return q[index].empty();
    }
};