232. Implement Queue using Stacks

难度: Easy

题目描述

Implement a first in first out (FIFO) queue using only two stacks. The implemented queue should support all the functions of a normal queue (push, peek, pop, and empty).

Implement the MyQueue class:

• void push(int x) Pushes element x to the back of the queue.
• int pop() Removes the element from the front of the queue and returns it.
• int peek() Returns the element at the front of the queue.
• boolean empty() Returns true if the queue is empty, false otherwise.

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, the stack may not be supported natively. You may simulate a stack using a list or deque (double-ended queue) as long as you use only a stack's standard operations.

 

Example 1:

Input
["MyQueue", "push", "push", "peek", "pop", "empty"]
[[], [1], [2], [], [], []]
Output
[null, null, null, 1, 1, false]

Explanation
MyQueue myQueue = new MyQueue();
myQueue.push(1); // queue is: [1]
myQueue.push(2); // queue is: [1, 2] (leftmost is front of the queue)
myQueue.peek(); // return 1
myQueue.pop(); // return 1, queue is [2]
myQueue.empty(); // return false

 

Constraints:

• 1 <= x <= 9
• At most 100 calls will be made to push, pop, peek, and empty.
• All the calls to pop and peek are valid.

 

Follow-up: Can you implement the queue such that each operation is amortized O(1) time complexity? In other words, performing n operations will take overall O(n) time even if one of those operations may take longer.

解题思路

代码实现

解决方案

java

class MyQueue {

    Stack<Integer> stk1 = null;
    Stack<Integer> stk2 = null;
    int top = 0;

    public MyQueue() {
        stk1 = new Stack<Integer>();
        stk2 = new Stack<Integer>();
        top = 0;
    }

    public void push(int x) {
        stk1.push(x);
        if(top==0){
            top=x;
        }
    }

    public int pop() {
        if(stk1.size()==0){
            return 0;
        }
        if(stk1.size()==1){
            top = 0;
            return stk1.pop();
        }
        while(stk1.size()>1){
            if(stk1.size()==2){
                top = stk1.peek();
            }
            stk2.push(stk1.pop());
        }
        int res = stk1.pop();
        while(stk2.size()>0){
            stk1.push(stk2.pop());
        }
        return res;

    }

    public int peek() {
        return top;
    }

    public boolean empty() {
        return stk1.size() == 0;
    }
}

/**
 * Your MyQueue object will be instantiated and called as such:
 * MyQueue obj = new MyQueue();
 * obj.push(x);
 * int param_2 = obj.pop();
 * int param_3 = obj.peek();
 * boolean param_4 = obj.empty();
 */