lc225

225-Implement Stack using Queue

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.

Example

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

Code

class MyStack {
public:
    queue<int> q1;
    queue<int> q2;
    int topV;

    MyStack() {
        
    }
    
    void push(int x) {
        q1.push(x);
        topV = x;
    }
    
    int pop() {
        while (q1.size() > 1) {
            int x = q1.front();
            q1.pop();
            q2.push(x); // Make a copy of q1
            topV = x;
        }

        int popValue = q1.front();
        q1.pop();

        swap(q1, q2);

        return popValue;
    }
    
    int top() {
        return topV;
    }
    
    bool empty() {
        return q1.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();
 */