算法通关村第一关|黄金挑战|链表中的环问题双向链表

1.链表中环的问题

1.1 判断是否有环：使用集合处理，判断是否碰撞。

public ListNode detectCycle(ListNode head) {ListNode pos = head;Set<ListNode> visited = new HashSet<ListNode>();while (pos != null) {if (visited.contains(pos)) {return pos;} else {visited.add(pos);}pos = pos.next;}return null;
}

1.2 判断是否有环：使用快慢指针，如果有环，慢的就一定能遇到快的。

public boolean hasCycle(ListNode head) {if (head == null || head.next == null) {return false;}ListNode fast = head;ListNode slow = head;while (fast != null && fast.next != null) {fast = fast.next.next;slow = slow.next;if (fast == slow) {return true;}}return false;
}

1.3 有环时判断环的位置：使用快慢指针。

public ListNode detectCycle(ListNode head) {if (head == null) {return null;}ListNode slow = head, fast = head;while (fast != null) {slow = slow.next;if (fast.next != null) {fast = fast.next.next;} else {return null;}if (fast == slow) {ListNode ptr = head;while (ptr != slow) {ptr = ptr.next;slow = slow.next;}return ptr;}}return null;
}

2.双向链表

2.1 双向链表的结点。

class DoubleNode {public int data;public DoubleNode next;public DoubleNode prev;public DoubleNode(int data) {this.data = data;}// 打印结点数据域public void displayNode() {System.out.println("{" + data + "}");}
}

2.2 双向链表的结构和遍历方法。

public class DoublyLinkList {// 分别指向双向链表的头部和尾部private DoubleNode first;private DoubleNode last;public DoublyLinkList() {first = null;last = first;}// 从头部开始打印public void displayForward() {System.out.println("List(first--->last):");DoubleNode current = first;while (current != null) {current.displayNode();current = current.next;}System.out.println();}// 从尾部开始演绎public void displayBackward() {System.out.println("List(last--->first):");DoubleNode current = last;while (current != null) {current.displayNode();current = current.prev;}System.out.println();}
}

2.3 双向链表插入元素：在头部和尾部插入元素。

// 头部插入
public void insertFirst(int data) {DoubleNode newDoubleNode = new DoubleNode(data);if (first == null) {last = newDoubleNode;} else {first.prev = newDoubleNode;}newDoubleNode.next = first;first = newDoubleNode;
}
//尾部插入
public void insertLast(int data) {DoubleNode newDoubleNode = new DoubleNode(data);if (first == null) {first = newDoubleNode;} else {newDoubleNode.prev = last;last.next = newDoubleNode;}last = newDoubleNode;
}

2.4 双向链表插入元素：在中间插入元素。

public void insertAfter(int key, int data) {DoubleNode newDoubleNode = new DoubleNode(data);DoubleNode current = first;while ((current != null) && (current.data != key)) {current = current.next;}// 当前结点current为空if (current == null) {if (first == null) {// 链表中没有元素，直接赋值first = newDoubleNode;last = newDoubleNode;} else {// 找不到key，在链表尾部插入新结点last.next = newDoubleNode;newDoubleNode.prev = last;last = newDoubleNode;}} else {// 链表中有元素，并且找到了keyif (current == last) {// key与最后结点的data相等newDoubleNode.next = null;last = newDoubleNode;} else {// 两结点中间插入newDoubleNode.next = current.next;current.next.prev = newDoubleNode;}current.next = newDoubleNode;newDoubleNode.prev = current;}
}

2.5 双向链表删除元素：删除首尾元素。

// 删除首部元素
public DoubleNode deleteFirst() {DoubleNode temp = first;if (first.next = null) {last = null;} else {first.next.prev = null;}first = first.next;return temp;
}// 删除尾部元素
public DoubleNode deleteLast() {DoubleNode temp = last;if (first.next == null) {first = null;} else {last.prev.next = null;}last = last.prev;return temp;
}

2.6 双向链表删除元素：删除中间元素。

public DoubleNode deleteKey(int key) {DoubleNode current = first;while (current != null && current.data != key) {current = current.next;}if (current == null) {return null;} else {if (current == first) {first = current.next;current.next.prev = null;} else if (current == last) {last = current.prev;current.prev.next = null;} else {current.prev.next = current.next;current.next.prev = current.prev;}}return current;
}