# Implement Queue Using Stacks Problem

## Description

LeetCode Problem 232.

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:

 1 2 3 4 5 6 7 8 9 10 11 12 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 100calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

## Sample C++ Code

 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 class MyQueue { public: stack s1, s2; /** Initialize your data structure here. */ MyQueue() { } /** Push element x to the back of queue. */ void push(int x) { s1.push(x); return; } /** Removes the element from in front of queue and returns that element. */ int pop() { int ele = 0; while (!s1.empty()) { s2.push(s1.top()); s1.pop(); } if (!s2.empty()) { ele = s2.top(); s2.pop(); } while (!s2.empty()) { s1.push(s2.top()); s2.pop(); } return ele; } /** Get the front element. */ int peek() { int ele = 0; while (!s1.empty()) { s2.push(s1.top()); s1.pop(); } if (!s2.empty()) { ele = s2.top(); } while (!s2.empty()) { s1.push(s2.top()); s2.pop(); } return ele; } /** Returns whether the queue is empty. */ bool empty() { return s1.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(); */