Leetcode 232. 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).

class Queue {
stack<int> input, output;
public:

void push(int x) {
input.push(x);
}

void pop(void) {
peek();
output.pop();
}

int peek(void) {
if (output.empty())
while (input.size())
output.push(input.top()), input.pop();
return output.top();
}

bool empty(void) {
return input.empty() && output.empty();
}
};