【代码随想录】链表-编程知识

203. 移除链表元素

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// public class ListNode {
//     int val;
//     ListNode next;
//     ListNode() {}
//     ListNode(int val) { this.val = val; }
//     ListNode(int val, ListNode next) { this.val = val; this.next = next;}
// }class Solution {public ListNode removeElements(ListNode head, int val) {// 在head之前创建一个虚拟节点ListNode preHead = new ListNode(0, head);ListNode current=preHead;while(current.next!=null){if(current.next.val == val){current.next=current.next.next;}else{current=current.next;}}return preHead.next;}
}

707.设计链表

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class ListNode {int val;ListNode next;public ListNode(int val) {this.val = val;}
}class MyLinkedList {int size;ListNode head;public MyLinkedList() {this.size=0;this.head=new ListNode(0);//头节点之前的节点}public int get(int index) {if(index<0 || index>=this.size){return -1;}   ListNode current=this.head;for(int i=0;i<=index;i++){current=current.next;}return current.val;}public void addAtHead(int val) {addAtIndex(0, val);    }public void addAtTail(int val) {addAtIndex(this.size, val);}public void addAtIndex(int index, int val) {if(index<0 || index>this.size){return ;}ListNode node = new ListNode(val);ListNode current=this.head;for(int i=0;i<index;i++){current=current.next;//最终指向index的前一个节点}node.next=current.next;current.next=node;size++;//新增元素后不要忘记链表长度+1}public void deleteAtIndex(int index) {if(index<0 || index>=this.size){return ;}ListNode current=this.head;for(int i=0;i<index;i++){current=current.next;//最终指向index的前一个节点}current.next=current.next.next;size--;//删除元素后不要忘记链表长度-1}
}

206.反转链表

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/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode reverseList(ListNode head) {if(head==null){return null;}ListNode pre=new ListNode(0, head);//添加首节点ListNode current=head;while(current!=null){ListNode post=current.next;current.next=pre;pre=current;current=post;}head.next=null;//去掉首节点return pre;}
}

24. 两两交换链表中的节点

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/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode swapPairs(ListNode head) {if(head==null || head.next==null){return head;}ListNode prehead=new ListNode(0, head);ListNode node1=head;ListNode node2=node1.next;ListNode pre=prehead;while(node1!=null && node2!=null){ListNode post=node2.next;pre.next=node2;node2.next=node1;node1.next=post;pre=node1;node1=post;if(node1!=null){node2=node1.next;}           }// 不能直接返回head，因为head现在指向链表的第二个元素return prehead.next;}
}

19.删除链表的倒数第 N 个节点

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/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode() {}*     ListNode(int val) { this.val = val; }*     ListNode(int val, ListNode next) { this.val = val; this.next = next; }* }*/
class Solution {public ListNode removeNthFromEnd(ListNode head, int n) {// 把preHead当成第0个，head当成第1个ListNode preHead=new ListNode(0, head);int length=0;ListNode pointer=preHead;while(pointer.next!=null){length++;pointer=pointer.next;}pointer=preHead;for(int i=1;i<length-n+1;i++){pointer=pointer.next;//最终指向要删除节点的前一个节点}pointer.next=pointer.next.next;// head节点有可能被删除了return preHead.next;}
}

160.相交链表

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/*** Definition for singly-linked list.* public class ListNode {*     int val;*     ListNode next;*     ListNode(int x) {*         val = x;*         next = null;*     }* }*/
public class Solution {public ListNode getIntersectionNode(ListNode headA, ListNode headB) {//存放链表A各节点的地址，目的是看链表B中是否有在set集合中的节点Set set = new HashSet();ListNode pointerA=headA;while(pointerA!=null){set.add(pointerA);pointerA=pointerA.next;}ListNode pointerB=headB;while(pointerB!=null && !set.contains(pointerB)){pointerB=pointerB.next;}//(1)在最后一个元素之后(null处)跳出循环//(2)在非空元素处跳出循环// if(pointerB==null){//     return null;// }else{//     pointerB;// }return pointerB;}
}

另一种解法：先遍历两个链表，得到它们的长度。然后，让长链表的指针先走它们长度差的步数，接着两个指针同时向前遍历，直到它们相遇或都为 null。

142.环形链表 II

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/*** Definition for singly-linked list.* class ListNode {*     int val;*     ListNode next;*     ListNode(int x) {*         val = x;*         next = null;*     }* }*/
public class Solution {public ListNode detectCycle(ListNode head) {Set set=new HashSet();ListNode pointer=head;while(!set.contains(pointer) && pointer!=null){set.add(pointer);pointer=pointer.next;}//(1)链表有环：因走到已经走过的节点而退出循环//(2)链表无环：因走到链表末尾而退出循环return pointer;}
}