# [leetcode] Implement Queue using Stacks

Implement Queue using Stacks

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).

Use stack.

Push – o(1)

pop – o(n)

peek – o(1)

empty – o(1)

```class Queue {
public:
stack<int> s;
int frontElement;
// Push element x to the back of queue.
void push(int x) {
if(s.empty()){
frontElement = x;
}
s.push(x);
}

// Removes the element from in front of queue.
void pop(void) {
stack<int> tmp;
while(!s.empty()){
tmp.push(s.top());
s.pop();
}
tmp.pop();
if(!tmp.empty()){
frontElement = tmp.top();
}
while(!tmp.empty()){
s.push(tmp.top());
tmp.pop();
}
}

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

// Return whether the queue is empty.
bool empty(void) {
return s.empty();
}
};```

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