剑指 Offer 09. 用两个栈实现队列

import java.util.Stack;

public class CQueue {
    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();
    public CQueue() {

    }

    public void appendTail(int value) {
        stack1.push(value);
    }

    public int deleteHead() {
        if(stack1.isEmpty()) return -1;
        while(!stack1.isEmpty()){
            stack2.push(stack1.pop());
        }
        int val = stack2.pop();
        while (!stack2.isEmpty()){
            stack1.push(stack2.pop());
        }
        return val;
    }
}

import java.util.Stack;

public class CQueue {
    Stack<Integer> stack1 = new Stack<>();
    Stack<Integer> stack2 = new Stack<>();
    public CQueue() {
        
    }

    public void appendTail(int value) {
        stack1.push(value);
    }

    public int deleteHead() {
        if(stack1.isEmpty() && stack2.isEmpty()) return -1;
        if(stack2.isEmpty()) {
            while (!stack1.isEmpty()) {
                stack2.push(stack1.pop());
            }
            return stack2.pop();
        }else{
            return stack2.pop();
        }
    }
}

 不用删除一个元素之后，就立刻将stack2中的元素复原到stack1中。