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).
Solution：Two stacks 倒一下
Time Complexity: O(1) amortized Space Complexity: O(N)
Solution1a Code：
class MyQueue {
Deque<Integer> input = new ArrayDeque();
Deque<Integer> output = new ArrayDeque();
public void push(int x) {
input.push(x);
}
public int pop() {
peek();
return output.pop();
}
public int peek() {
if (output.isEmpty())
while (!input.isEmpty()) {
output.push(input.pop());
}
return output.peek();
}
public boolean empty() {
return input.isEmpty() && output.isEmpty();
}
}
