# 232. Implement Queue using Stacks

Difficulty: Easy

Frequency: N/A

Implement the following operations of a queue using stacks.

• push(x) — Push element x to the back of queue.
• pop() — Removes the element from in front of queue.
• peek() — Get the front element.
• empty() — Return whether the queue is empty.

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, stack may not be supported natively. You may simulate a stack by using a list or deque (double-ended queue), as long as you use only standard operations of a stack.
• You may assume that all operations are valid (for example, no pop or peek operations will be called on an empty queue).

My solution:
Data structure:
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Code:
```class Queue {
public:
// Push element x to the back of queue.
void push(int x) {
stack<int> tmp;
while (!s.empty()) {
tmp.push(s.top());
s.pop();
}
s.push(x);
while (!tmp.empty()) {
s.push(tmp.top());
tmp.pop();
}
}

// Removes the element from in front of queue.
void pop(void) {
s.pop();
}

// Get the front element.
int peek(void) {
return s.top();
}

// Return whether the queue is empty.
bool empty(void) {
return s.empty();
}
private:
stack<int> s;
};
```

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# 225. Implement Stack using Queues

Difficulty: Easy

Frequency: N/A

Implement the following operations of a stack using queues.

• push(x) — Push element x onto stack.
• pop() — Removes the element on top of the stack.
• top() — Get the top element.
• empty() — Return whether the stack is empty.

Notes:

• You must use only standard operations of a queue — which means only `push to back`, `peek/pop from front`, `size`, and `is empty` operations are valid.
• Depending on your language, queue may not be supported natively. You may simulate a queue by using a list or deque (double-ended queue), as long as you use only standard operations of a queue.
• You may assume that all operations are valid (for example, no pop or top operations will be called on an empty stack).

My solution:
Data structure:
Steps:
Complexity:
Runtime:
Space:
Test cases:
Corner cases:
Code:
```class Stack {
public:
// Push element x onto stack.
void push(int x) {
queue<int> tmp;
while (!q.empty()) {
tmp.push(q.front());
q.pop();
}
q.push(x);
while(!tmp.empty()) {
q.push(tmp.front());
tmp.pop();
}
}

// Removes the element on top of the stack.
void pop() {
q.pop();
}

// Get the top element.
int top() {
return q.front();
}

// Return whether the stack is empty.
bool empty() {
return q.empty();
}
private:
queue<int> q;
};
```

Another solution:
Data structure:
steps:
Complexity:
Runtime:
Space:
Code:

Things to learn: