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前言

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一、21. 合并两个有序链表

/*** 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 mergeTwoLists(ListNode list1, ListNode list2) {ListNode res = new ListNode(), pre = res;ListNode p1 = list1, p2 = list2;while(p1 != null && p2 != null){if(p1.val <= p2.val){pre.next = p1;p1 = p1.next;pre = pre.next; }else{pre.next = p2;p2 = p2.next;pre = pre.next;}}while(p1 != null){pre.next = p1;p1 = p1.next;pre = pre.next;}while(p2 != null){pre.next = p2;p2 = p2.next;pre = pre.next;}return res.next;}
}

二、力扣86. 分隔链表

/*** 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 partition(ListNode head, int x) {ListNode dump1 = new ListNode(-1,null);ListNode dump2 = new ListNode(-1,null);ListNode p = head, p1 = dump1, p2 = dump2;while(p != null){if(p.val < x){p1.next = p;p1 = p1.next;}else{p2.next = p;p2 = p2.next;}ListNode temp = p.next;p.next = null;p = temp;}p1.next = dump2.next;return dump1.next;}
}

三、力扣23. 合并 K 个升序链表

/*** 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 mergeKLists(ListNode[] lists) {if(lists.length == 0){return null;}ListNode res = new ListNode(-1,null);ListNode r = res;PriorityQueue<ListNode> pq = new PriorityQueue<>(lists.length, (a,b)->(a.val - b.val));for(ListNode node : lists){if(node != null){pq.add(node);}}while(!pq.isEmpty()){ListNode p = pq.poll();r.next = p;if(p.next != null){pq.add(p.next);}r = r.next;}return res.next;}
}

四、力扣删除链表的倒数第 N 个结点

/*** 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) {ListNode p1 = head;for(int i = 0; i < n; i ++){p1 = p1.next;}if(p1 == null){return head.next;}ListNode p2 = head;while(p1.next != null){p1 = p1.next;p2 = p2.next;}p2.next = p2.next.next;return head;}
}