232 Implement Queue using Stacks
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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 onlypush to top
, peek/pop from top, size
, andis 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).
思路:主要是需要两个stack来保证输入的顺序和queue的一致
所有操作的时间复杂度都是O(n), 空间复杂度是O(n)