Leetcode.com/problems/im…

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.

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.

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
Copy the code

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.

Solution a:

Use two stacks.

class MyQueue() { /** Initialize your data structure here. */ var stack1 = Stack<Int>() var stack2 = Stack<Int>() /** Push element x to the back of queue. */ fun push(x: Int) { stack1.push(x) } /** Removes the element from in front of queue and returns that element. */ fun pop(): Int { if (stack2.empty()) { while (! stack1.empty()) { val temp = stack1.pop() stack2.push(temp) } } return stack2.pop() } /** Get the front element. */ fun peek(): Int { if (stack2.empty()) { while (! stack1.empty()) { val temp = stack1.pop() stack2.push(temp) } } return stack2.peek() } /** Returns whether the queue is empty. */ fun empty(): Boolean { return stack1.empty() && stack2.empty() } } /** * Your MyQueue object will be instantiated and called as such:  * var obj = MyQueue() * obj.push(x) * var param_2 = obj.pop() * var param_3 = obj.peek() * var param_4 = obj.empty() */Copy the code