Implement Stack using Queues

Description

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

• You must use only standard operations of a queue, which means that only push to back, peek/pop from front, size and is empty operations are valid.
• Depending on your language, the queue may not be supported natively. You may simulate a queue using a list or deque (double-ended queue) as long as you use only a queue’s standard operations.

Example 1:

Input [“MyStack”, “push”, “push”, “top”, “pop”, “empty”] [[], [1], [2], [], [], []] Output [null, null, null, 2, 2, false]

Explanation MyStack myStack = new MyStack(); myStack.push(1); myStack.push(2); myStack.top(); // return 2 myStack.pop(); // return 2 myStack.empty(); // return False

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid.

Follow-up: Can you implement the stack using only one queue?

Code

Using 2 queues

class MyStack {
public:
    MyStack() {
        
    }
    
    void push(int x) {
        auxiliary_queue.push(x);
        while(!main_queue.empty()) {
            auxiliary_queue.push(main_queue.front());
            main_queue.pop();
        }
        swap(main_queue, auxiliary_queue);
    }
    
    int pop() {
        int result = top();
        main_queue.pop();
        return result;
    }
    
    int top() {
        return main_queue.front();
    }
    
    bool empty() {
        return main_queue.empty() && auxiliary_queue.empty();
    }
 
private:
    queue<int> main_queue;
    queue<int> auxiliary_queue;
};
 
 
 
/**
 * 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();
 */

Using 1 queue

Rotating.

class MyStack {
public:
    MyStack() {
        
    }
    
    void push(int x) {
        main_queue.push(x);
        for(int i = 0; i < main_queue.size() - 1; i++) {
            main_queue.push(main_queue.front());
            main_queue.pop();
        }
    }
    
    int pop() {
        int result = top();
        main_queue.pop();
        return result;
    }
    
    int top() {
        return main_queue.front();
    }
    
    bool empty() {
        return main_queue.empty();
    }
 
private:
    queue<int> main_queue;
};
 
 
 
/**
 * 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();
 */

Source

• Implement Stack using Queues - LeetCode