225. Implement Stack using Queues

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225. Implement Stack using Queues

Description

Difficulty: Easy

Related Topics: Stack, Design, Queue

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:

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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?

Hints/Notes

  • pop and re-enqueue all previous elements when trying to access the last element

Solution

Language: C++

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class MyStack {
public:
    queue<int> q;
    int head;

    MyStack() {
        
    }
    
    void push(int x) {
        head = x;
        q.push(x);
    }
    
    int pop() {
        int size = q.size();
        for (int i = 0; i < size - 1; i++) {
            int num = q.front();
            q.pop();
            q.push(num);
            head = num;
        }
        int res = q.front();
        q.pop();
        return res;
    }
    
    int top() {
        return head;
    }
    
    bool empty() {
        return q.empty();   
    }
};

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