剑指09

剑指 Offer 09. 用两个栈实现队列

题目：

思路：

两个栈：
一个进栈，一个出栈
1.进栈负责push
2.当出栈空，进栈不为空时，将进栈中的pop，push到出栈。。再pop
3.都为空时返回-1；

代码：

class CQueue {

    private Stack<Integer> stack1 = new Stack();
    private Stack<Integer> stack2 = new Stack();

    public CQueue() {
    }

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

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

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