剑指 Offer 09. 用两个栈实现队列

Posted on | In
Words count in article: 223 | Reading time ≈ 1

题目

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:

1
2
3
4
输入：
["CQueue","appendTail","deleteHead","deleteHead"]
[[],[3],[],[]]
输出：[null,null,3,-1]

示例2：

1
2
3
4
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

解法

解法一：

JAVA

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
class CQueue {
        private Stack<Integer> s1;
        private Stack<Integer> s2;
        public CQueue() {
            s1 = new Stack<>();
            s2 = new Stack<>();
        }

        public void appendTail(int value) {
            s2.push(value);
        }

        public int deleteHead() {
            if (s1.empty()) {
                while (!s2.empty()) {
                    s1.push(s2.pop());
                }
                if (s1.empty()) {
                    return -1;
                }
            }
            return s1.pop();
        }
    }
0%