Implement Queue using Stacks

描述

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).

分析

• push(x)，先将s中的元素全部弹出来，存入tmp，把x push 到tmp，然后把tmp中的元素全部弹出来，存入s
• pop()，直接将s的栈顶元素弹出来即可

代码

// Implement Queue using Stacks
class MyQueue {
// Push element x to the back of queue.
// Time Complexity: O(n)
public void push(int x) {
while (!s.isEmpty()) {
final int e = s.pop();
tmp.push(e);
}
tmp.push(x);

while(!tmp.isEmpty()) {
final int e = tmp.pop();
s.push(e);
}
}

// Removes the element from in front of queue.
// Time Complexity: O(1)
public void pop() {
s.pop();
}

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

// Return whether the queue is empty.
public boolean empty() {
return s.isEmpty();
}

private final Stack s = new Stack<>();
private final Stack tmp = new Stack<>();
}