🌗 225. 用队列实现栈

难度: 🌗

问题描述

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解法 1 - 2 个队列

class MyStack {
    LinkedList<Integer> queue1;
    LinkedList<Integer> queue2;

    public MyStack() {
        queue1 = new LinkedList<>();
        queue2 = new LinkedList<>();
    }
    
    public void push(int x) {
        queue1.offer(x);
    }
    
    public int pop() {
        // queue1 依次从队头弹出到 queue2
        int res = 0;
        while(!queue1.isEmpty()) {
            res = queue1.poll();
            if(!queue1.isEmpty()) {
                queue2.offer(res);
            }
        }
        while(!queue2.isEmpty()) {
            queue1.offer(queue2.poll());
        }
        return res;
    }
    
    public int top() {
        // queue1 依次从队头弹出到 queue2
        int res = 0;
        while(!queue1.isEmpty()) {
            res = queue1.poll();
            queue2.offer(res);
        }
        while(!queue2.isEmpty()) {
            queue1.offer(queue2.poll());
        }
        return res;
    }
    
    public boolean empty() {
        return queue1.isEmpty();
    }
}

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

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输出 1

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解法 2 - [进阶] 通过 1 个队列

class MyStack {
    LinkedList<Integer> queue1;

    public MyStack() {
        queue1 = new LinkedList<>();
    }
    
    public void push(int x) {
        queue1.offer(x);
    }
    
    public int pop() {
        // 获取当前队列中元素个数，依次取出 n - 1 个放入队列尾
        int size = queue1.size();
        for(int i = 0; i < size - 1; i ++) {
            queue1.offer(queue1.poll());
        }
        int res = queue1.poll();
        return res;
    }
    
    public int top() {
        // 获取当前队列中元素个数，依次取出 n - 1 个放入队列尾
        int size = queue1.size();
        for(int i = 0; i < size - 1; i ++) {
            queue1.offer(queue1.poll());
        }
        int res = queue1.poll();
        queue1.offer(res);
        return res;
    }
    
    public boolean empty() {
        return queue1.isEmpty();
    }
}

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

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输出