代码随想录算法训练营第十天【栈与队列】232.用栈实现队列、225.用队列实现栈

232.用栈实现队列

力扣题目链接

心得：思路不难，栈和队列的特性要掌握

　　1）栈，先进后出；队列，先进先出

　　2）初始化一个栈，Stack<Integer> stack = new Stack<>()

　　3）放入元素，stack.push()

　　4）弹出栈顶元素，stack.pop()

　　5）获取栈顶元素但不弹出，stack.peek()

　　6）判断栈是否为空，stack.isEmpty()

class MyQueue {
    Stack<Integer> stackIn;
    Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();
        stackOut = new Stack<>();
    }
    
    public void push(int x) {
        stackIn.push(x);
    }
    
    public int pop() {
        if(stackOut.isEmpty()){
            while(!stackIn.isEmpty()){
                stackOut.push(stackIn.pop());
            }
        }
        return stackOut.pop();
    }
    
    public int peek() {
        int result = this.pop();
        stackOut.push(result);
        return result;
    }
    
    public boolean empty() {
        return stackIn.isEmpty() && stackOut.isEmpty();
    }
}

/**
 * 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();
 */

225.用队列实现栈

力扣题目链接

心得：掌握队列的基本方法，关注pop()的实现方式

　　1）在队列后面增加x，deque.addLast(x)

　　2）获取队列的第一个值，但不弹出，deque.peekFirst()

　　3）弹出队列的第一个值，deque.pollFirst()

class MyStack {
    Deque<Integer> deque;
    public MyStack() {
        deque = new ArrayDeque<>();
    }
    
    public void push(int x) {
        deque.addLast(x);
    }
    
    public int pop() {
        int move = deque.size()-1;
        while (move-- > 0) {
            deque.addLast(deque.pollFirst());
        }
        return deque.pollFirst();
    }
    
    public int top() {
        return deque.peekLast();
    }
    
    public boolean empty() {
        return deque.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();
 */