如何用JAVA程式來查詢連結列表是否包含迴圈
摘要：
查詢連結串列是否包含迴圈的演算法
迭代連結串列時使用快速和慢速兩個指標。快速指標在每次迭代中移動兩個節點，而慢速指標移動到一個節點。如果連結串列包含迴圈或迴圈，那麼在迭代過程中，快指標和慢指標都會在某個點上相遇。如果它們不相交，並且快速或慢速將指向空，那麼連結串列就不是迴圈的，它不包含任何...
查詢連結串列是否包含迴圈的演算法
迭代連結串列時使用快速和慢速兩個指標。快速指標在每次迭代中移動兩個節點，而慢速指標移動到一個節點。如果連結串列包含迴圈或迴圈，那麼在迭代過程中，快指標和慢指標都會在某個點上相遇。如果它們不相交，並且快速或慢速將指向空，那麼連結串列就不是迴圈的，它不包含任何迴圈。這是精確的演算法
1）使用快速和慢速兩個指標
2）每次迭代快速移動兩個節點，慢速移動一個節點
3）如果快速和慢速相遇，則連結串列包含迴圈
4）如果fast指向空或fast.next指向空，則連結串列不是迴圈的。
下一部分包含Java程式來檢查連結串列是否包含迴圈，這是上述演算法的精確實現。該演算法也被稱為Floyd的迴圈查詢演算法，通常被稱為Tortoise and Hare演算法，用於查詢連結串列中的迴圈。
Java程式檢查連結串列是否為迴圈
這個Java程式使用LinkedList（不Java.UTI.LIKEDLIST）和連結串列的前一個節點類，修改了新增ToStTrn（）方法和AppEntoTead（）方法。另外，連結串列的iscyclic（）方法用於實現邏輯，以確定連結串列是否包含迴圈。隨後is cyclic（）如果連結串列是迴圈的，則返回true，否則返回false。
<font><i>/*
* Java class to represent linked list data structure.
*/</i></font><font>
<b>public</b> <b>class</b> LinkedList {
<b>private</b> Node head;
<b>public</b> LinkedList() { <b>this</b>.head = <b>new</b> Node(</font><font>"head"</font><font>); }
<b>public</b> Node head() { <b>return</b> head;}
<b>public</b> <b>void</b> appendIntoTail(Node node) {
Node current = head;
</font><font><i>//find last element of LinkedList i.e. tail</i></font><font>
<b>while</b>(current.next() != <b>null</b>){
current = current.next();
}
</font><font><i>//appending new node to tail in LinkedList</i></font><font>
current.setNext(node);
}
</font><font><i>/*
* If singly LinkedList contains Cycle then following would be true
* 1) slow and fast will point to same node i.e. they meet
* On the other hand if fast will point to null or next node of
* fast will point to null then LinkedList does not contains cycle.
*/</i></font><font>
<b>public</b> <b>boolean</b> isCyclic(){
Node fast = head;
Node slow = head;
<b>while</b>(fast!= <b>null</b> && fast.next != <b>null</b>){
fast = fast.next.next;
slow = slow.next;
</font><font><i>//if fast and slow pointers are meeting then LinkedList is cyclic</i></font><font>
<b>if</b>(fast == slow ){
<b>return</b> <b>true</b>;
}
}
<b>return</b> false;
}
@Override
<b>public</b> String toString(){
StringBuilder sb = <b>new</b> StringBuilder();
Node current = head.next();
<b>while</b>(current != <b>null</b>){
sb.append(current).append(</font><font>"-->"</font><font>);
current = current.next();
}
sb.delete(sb.length() - 3, sb.length()); </font><font><i>// to remove --> from last node</i></font><font>
<b>return</b> sb.toString();
}
<b>public</b> <b>static</b> <b>class</b> Node {
<b>private</b> Node next;
<b>private</b> String data;
<b>public</b> Node(String data) {
<b>this</b>.data = data;
}
<b>public</b> String data() { <b>return</b> data; }
<b>public</b> <b>void</b> setData(String data) { <b>this</b>.data = data;}
<b>public</b> Node next() { <b>return</b> next; }
<b>public</b> <b>void</b> setNext(Node next) { <b>this</b>.next = next; }
@Override
<b>public</b> String toString() {
<b>return</b> <b>this</b>.data;
}
}
}
</font>
測試迴圈的連結串列
在本節中，我們將使用帶有兩個連結串列的Java main方法測試，一個包含迴圈而另一個不迴圈。 您甚至可以為isCyclic（）方法編寫JUnit測試用例，以測試不同位置的迴圈的不同連結串列。 這是連結串列不包含任何迴圈的第一個測試。
<font><i>/**
*
* Java program to find if LinkedList contains loop or cycle or not.
* This example uses two pointer approach to detect cycle in linked list.
* Fast pointer moves two node at a time while slow pointer moves one node.
* If linked list contains any cycle or loop then both pointer will meet some time.
*
* @author Javin Paul
*/</i></font><font>
<b>public</b> <b>class</b> LinkedListTest {
<b>public</b> <b>static</b> <b>void</b> main(String args[]) {
</font><font><i>//creating LinkedList with 5 elements including head</i></font><font>
LinkedList linkedList = <b>new</b> LinkedList();
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"101"</font><font>));
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"201"</font><font>));
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"301"</font><font>));
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"401"</font><font>));
System.out.println(</font><font>"Linked List : "</font><font> + linkedList);
<b>if</b>(linkedList.isCyclic()){
System.out.println(</font><font>"Linked List is cyclic as it contains cycles or loop"</font><font>);
}<b>else</b>{
System.out.println(</font><font>"LinkedList is not cyclic, no loop or cycle found"</font><font>);
}
}
}
Output:
Linked List : 101-->201-->301-->401
LinkedList is not cyclic, no loop or cycle found
</font>
現在讓我們改變連結串列，使其包含迴圈
<font><i>//creating LinkedList with 5 elements including head</i></font><font>
LinkedList linkedList = <b>new</b> LinkedList();
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"101"</font><font>));
LinkedList.Node cycle = <b>new</b> LinkedList.Node(</font><font>"201"</font><font>);
linkedList.appendIntoTail(cycle);
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"301"</font><font>));
linkedList.appendIntoTail(<b>new</b> LinkedList.Node(</font><font>"401"</font><font>));
linkedList.appendIntoTail(cycle);
</font><font><i>//don't call toString method in case of cyclic linked list, it will throw OutOfMemoryError</i></font><font>
</font><font><i>//System.out.println("Linked List : " + linkedList);</i></font><font>
<b>if</b>(linkedList.isCyclic()){
System.out.println(</font><font>"Linked List is cyclic as it contains cycles or loop"</font><font>);
}<b>else</b>{
System.out.println(</font><font>"LinkedList is not cyclic, no loop or cycle found"</font><font>);
}
Output:
Linked List is cyclic as it contains cycles or loop
</font>