Problem

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).

Example

MyStack stack = new MyStack();

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

Code

static int var = [](){
    std::ios::sync_with_stdio(false);
    cin.tie(NULL);
    return 0;
}();
class MyStack {
private:
    queue<int> stack;
public:
    /** 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 temp = 0;
        for(int i=0;i<stack.size()-1;i++){
            temp = stack.front();
            stack.pop();
            stack.push(temp);
        }
        temp = stack.front();
        stack.pop();
        return temp;
    }
    
    /** Get the top element. */
    int top() {
        int temp = 0;
        for(int i=0;i<stack.size()-1;i++){
            temp = stack.front();
            stack.pop();
            stack.push(temp);
        }
        temp = stack.front();
        stack.pop();
        stack.push(temp);
        return temp;
    }
    
    /** Returns whether the stack is empty. */
    bool empty() {
        return stack.empty();
    }
};

/**
 * 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();
 */

Result

225. Implement Stack using Queues.png