notes/OJ notes/pages/Leetcode Implement-Queue-Using-Stacks.md

Leetcode Implement-Queue-Using-Stacks

2022-07-03 09:52

Data structures:

#DS #stack #queue

Difficulty:

#coding_problem #difficulty_easy

Additional tags:

#leetcode #CS_list_need_understanding

Revisions:

2022-09-21

Related topics:

Links:

• Link to problem

---

Problem

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.

Examples

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

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.

Thoughts

[!summary] Use two stacks to implement a queue.

One stack reverse the FIFO order, but two stack can reverse back.

In real life, this can improve speed as you only lock one stack because of thread safety.

Solution

class MyQueue {
  stack<int> input;
  stack<int> out;

public:
  void push(int x) { input.push(x); }

  int pop() {
    move();
    int top = out.top();
    out.pop();
    return top;
  }

  int peek() {
    move();
    return out.top();
  }

  void move() {
    if (out.empty()) {
      while (!input.empty()) {
        out.push(input.top());
        input.pop();
      }
    }
  }

  bool empty() { return input.empty() && out.empty(); }
};

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue* obj = new MyQueue();
 * obj->push(x);
 * int param_2 = obj->pop();
 * int param_3 = obj->peek();
 * bool param_4 = obj->empty();
 */