# Implement Stack Using Queues Problem

## Description

LeetCode Problem 225.

Implement a last-in-first-out (LIFO) stack using only two queues. The implemented stack should support all the functions of a normal stack (push, top, pop, and empty).

Implement the MyStack class:

• void push(int x) Pushes element x to the top of the stack.
• int pop() Removes the element on the top of the stack and returns it.
• int top() Returns the element on the top of the stack.
• boolean empty() Returns true if the stack is empty, false otherwise.

Notes:

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

Example 1:

``````1
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Input
["MyStack", "push", "push", "top", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 2, 2, false]
Explanation
MyStack myStack = new MyStack();
myStack.push(1);
myStack.push(2);
myStack.top(); // return 2
myStack.pop(); // return 2
myStack.empty(); // return False
``````

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, top, and empty.
• All the calls to pop and top are valid. Follow-up: Can you implement the stack using only one queue?

## Sample C++ Code

``````1
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class MyStack {
public:
queue<int> q1, q2;

/** Initialize your data structure here. */
MyStack() {

}

/** Push element x onto stack. */
void push(int x) {
q1.push(x);
}

/** Removes the element on top of the stack and returns that element. */
int pop() {
int ele = 0;
int cnt = 0;
while (!q1.empty()) {
ele = q1.front();
q2.push(ele);
q1.pop();
cnt ++;
}
while (!q2.empty()) {
if (cnt > 1) {
q1.push(q2.front());
q2.pop();
cnt --;
} else {
q2.pop();
}
}
return ele;
}

/** Get the top element. */
int top() {
int ele = 0;
while (!q1.empty()) {
ele = q1.front();
q2.push(ele);
q1.pop();
}
while (!q2.empty()) {
q1.push(q2.front());
q2.pop();
}
return ele;
}

/** Returns whether the stack is empty. */
bool empty() {
return (q1.empty());
}
};

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