剑指offer_05:用两个栈实现队列

用两个栈实现一个队列。队列的声明如下，请实现它的两个函数 appendTail 和 deleteHead ，分别完成在队列尾部插入整数和在队列头部删除整数的功能。(若队列中没有元素，deleteHead 操作返回 -1 )

示例 1：
输入：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
输出：[null,null,3,-1]

示例 2：
输入：
["CQueue","deleteHead","appendTail","appendTail","deleteHead","deleteHead"]
[[],[],[5],[2],[],[]]
输出：[null,-1,null,null,5,2]

提示：
1 <= values <= 10000
最多会对 appendTail、deleteHead 进行 10000 次调用

1、原始想法

class CQueue {
    Stack<Integer> stack;
    Stack<Integer> helper;
    public CQueue() {
        stack=new Stack<Integer>();
        helper=new Stack<Integer>();
    }
    
    public void appendTail(int value) {
        stack.push(value);
    }
    
    public int deleteHead() {
        if(stack.isEmpty()){
            return -1;
        }
        while(!stack.isEmpty()){
            helper.push(stack.pop());
        }
        int res=helper.pop();
        while(!helper.isEmpty()){
            stack.push(helper.pop());
        }
        return res;
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */

2、优化

stack的底层是数组，有扩容和移位的问题。可以使用LinkedList来实现。

class CQueue {
    LinkedList<Integer> stack;
    LinkedList<Integer> helper;
    public CQueue() {
        stack=new LinkedList<Integer>();
        helper=new LinkedList<Integer>();
    }
    
    public void appendTail(int value) {
        stack.addLast(value);
    }
    
    public int deleteHead() {
        if(!helper.isEmpty()){
            return helper.removeLast();
        }
        if(stack.isEmpty()){
            return -1;
        }else{
            while(!stack.isEmpty()){
                helper.addLast(stack.removeLast());
            }
            return helper.removeLast();
        }
    }
}

/**
 * Your CQueue object will be instantiated and called as such:
 * CQueue obj = new CQueue();
 * obj.appendTail(value);
 * int param_2 = obj.deleteHead();
 */