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]
解题思路：
package Algriothm;
import java.util.Stack;
public class CQueue {
public static void main(String[] args) {
CQueue cQueue = new CQueue();
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
cQueue.appendTail(20);
System.out.println(cQueue.deleteHead());
cQueue.appendTail(1);
cQueue.appendTail(8);
cQueue.appendTail(20);
cQueue.appendTail(1);
cQueue.appendTail(11);
cQueue.appendTail(2);
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
System.out.println(cQueue.deleteHead());
}
Stack<Integer> stack1;
Stack<Integer> stack2;
public CQueue() {
stack1 = new Stack();
stack2 = new Stack();
}
public void appendTail(int value) {
stack1.push(value);
}
public int deleteHead() {
if (stack2.size() != 0) {
return stack2.pop();
}
if (stack1.size() == 0) {
return -1;
}
while (stack1.size() != 0) {
stack2.push(stack1.pop());
}
return stack2.pop();
}
}
原文：https://www.cnblogs.com/yssd/p/15155725.html