Today I have done leetcode's January LeetCoding Challenge with cpp.

January LeetCoding Challenge 29

Description

Implement Queue using Stacks

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

Notes:

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

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

Constraints:

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

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

Solution

auto speedup = [](){
  cin.tie(nullptr);
  cout.tie(nullptr);
  ios::sync_with_stdio(false);
  return 0;
}();
class MyQueue {
  vector<int> out;
  vector<int> in;

  void refill() {
    if(out.size()) return;
    while(in.size()) {
      out.push_back(in.back());
      in.pop_back();
    }
  }
public:
  /** Initialize your data structure here. */
  MyQueue() {}
  
  /** Push element x to the back of queue. */
  void push(int x) {
    in.push_back(x);
  }
  
  /** Removes the element from in front of queue and returns that element. */
  int pop() {
    refill();
    int val = out.back();
    out.pop_back();
    return val;
  }
  
  /** Get the front element. */
  int peek() {
    refill();
    return out.back();
  }
  
  /** Returns whether the queue is empty. */
  bool empty() {
    return out.empty() && in.empty();
  }
};