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

可以用两个栈，s和tmp，s存放元素，tmp用来作中转。

• 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

该算法push的算法复杂度是O(n), pop的算法复杂度O(1)。

另个一个方法是，让pop是O(n), push是O(1)，思路很类似，就不赘述了。

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<>();
}

相关题目

• Implement Stack using Queues