34 lines
1.2 KiB
Java
34 lines
1.2 KiB
Java
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# 9. 用两个栈实现队列
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[NowCoder](https://www.nowcoder.com/practice/54275ddae22f475981afa2244dd448c6?tpId=13&tqId=11158&tPage=1&rp=1&ru=/ta/coding-interviews&qru=/ta/coding-interviews/question-ranking&from=cyc_github)
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## 题目描述
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用两个栈来实现一个队列，完成队列的 Push 和 Pop 操作。
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## 解题思路
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in 栈用来处理入栈（push）操作，out 栈用来处理出栈（pop）操作。一个元素进入 in 栈之后，出栈的顺序被反转。当元素要出栈时，需要先进入 out 栈，此时元素出栈顺序再一次被反转，因此出栈顺序就和最开始入栈顺序是相同的，先进入的元素先退出，这就是队列的顺序。
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<img src="https://cs-notes-1256109796.cos.ap-guangzhou.myqcloud.com/3ea280b5-be7d-471b-ac76-ff020384357c.gif" width="350"/>
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```java
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Stack<Integer> in = new Stack<Integer>();
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Stack<Integer> out = new Stack<Integer>();
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public void push(int node) {
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in.push(node);
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}
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public int pop() throws Exception {
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if (out.isEmpty())
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while (!in.isEmpty())
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out.push(in.pop());
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if (out.isEmpty())
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throw new Exception("queue is empty");
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return out.pop();
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}
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```
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