232.用栈实现队列

我的解法

class MyQueue {

    Stack<Integer> stackIn;
    Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();// 负责进栈
        stackOut = new Stack<>();// 负责出栈
    }

    public void push(int x) {
        // 把出栈元素全部放到进栈
        while (!stackOut.empty()) {
            stackIn.push(stackOut.pop());
        }
        stackIn.push(x);
    }

    public int pop() {
        // 把进栈元素全部放到出栈
        while (!stackIn.empty()) {
            stackOut.push(stackIn.pop());
        }
        return stackOut.pop();
    }

    public int peek() {
        // 应该返回出栈的栈顶
        while (!stackIn.empty()) {
            stackOut.push(stackIn.pop());
        }
        return stackOut.peek();
    }

    public boolean empty() {
        return stackIn.empty() && stackOut.empty();
    }
}

/**
 * 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();
 */

结合官方代码

class MyQueue {

    Stack<Integer> stackIn;
    Stack<Integer> stackOut;

    public MyQueue() {
        stackIn = new Stack<>();// 负责进栈
        stackOut = new Stack<>();// 负责出栈
    }

    public void push(int x) {
        stackIn.push(x);
    }

    public int pop() {
        dumpStackIn();
        return stackOut.pop();
    }

    public int peek() {
        // 应该返回出栈的栈顶
        dumpStackIn();
        return stackOut.peek();
    }

    public boolean empty() {
        return stackIn.empty() && stackOut.empty();
    }

    private void dumpStackIn() {
        // 把进栈元素全部放到出栈
        //如果出栈还有元素就不需要 
        if (!stackOut.empty())
            return;
        while (!stackIn.empty()) {
            stackOut.push(stackIn.pop());
        }
    }
}

/**
 * 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();
 */

可改进的部分

在出栈的时候如果stackOut中有元素，则直接可以弹出；如果没有就把所有的stackIn元素倒入stackOut。
这样的话进栈的时候直接压入stackIn就可以。