Question

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.

Example:

MyStack stack = new MyStack();

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

Notes:

• You must use only standard operations of a queue -- which means only push to back, peek/pop from front, size, and is emptyoperations 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).

Difficulty:Easy

Category:Stack, Design

Analyze

Solution

class MyStack {
 public:
  /** Initialize your data structure here. */
  MyStack() {}

  /** Push element x onto stack. */
  void push(int x) {
    q_.push(x);
    for (int i = 0; i < q_.size() - 1; ++i) {
      q_.push(q_.front());
      q_.pop();
    }
  }

  /** Removes the element on top of the stack and returns that element. */
  int pop() {
    int temp = q_.front();
    q_.pop();
    return temp;
  }

  /** Get the top element. */
  int top() { return q_.front(); }

  /** Returns whether the stack is empty. */
  bool empty() { return q_.empty(); }

 private:
  queue<int> q_;
};