21. Merge Two Sorted Lists★
题目内容:Merge two sorted linked lists and return it as a new list. The new list should be made by splicing together the nodes of the first two lists.
题目分析:本题是要合并两个已经有序的单链表,思路很简单,有两种方法:非递归和递归。
题目代码:
(1)非递归:
为方便操作,定义一个辅助的头节点,然后比较原来两个链表的头节点,将小的那一个加入到合并链表,最后,当其中一个链表为空时,直接将另一个链表接入到合并链表即可。
//public class LeetCode21 为测试
public class LeetCode21 {
public static void main(String[] args) {
ListNode m1=new ListNode(1),m2=new ListNode(3),m3=new ListNode(5);
m1.next=m2;
m2.next=m3;
System.out.println("链表1:"+m1.val+"->"+m2.val+"->"+m3.val);
ListNode n1=new ListNode(2),n2=new ListNode(4),n3=new ListNode(6);
n1.next=n2;
n2.next=n3;
System.out.println("链表2:"+n1.val+"->"+n2.val+"->"+n3.val);
ListNode result=new Solution().mergeTwoLists(m1, n1);
if(result!=null){
System.out.print("合并链表:"+result.val);
ListNode resultNext=result.next;
while(resultNext!=null){
System.out.print("->"+resultNext.val);
resultNext=resultNext.next;
}
}
}
}
class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode fakeHead=new ListNode(0);
ListNode p=fakeHead;
while(l1!=null&&l2!=null){
if(l1.val<l2.val){
p.next=l1;
l1=l1.next;
}else{
p.next=l2;
l2=l2.next;
}
p=p.next;
}
if(l1!=null) p.next=l1;
if(l2!=null) p.next=l2;
return fakeHead.next;
}
}
class ListNode {
int val;
ListNode next;
ListNode(int x) { val = x; }
}
(2)递归:
//public class LeetCode21 为测试
public class LeetCode21 {
public static void main(String[] args) {
ListNode m1=new ListNode(1),m2=new ListNode(3),m3=new ListNode(5);
m1.next=m2;
m2.next=m3;
System.out.println("链表1:"+m1.val+"->"+m2.val+"->"+m3.val);
ListNode n1=new ListNode(2),n2=new ListNode(4),n3=new ListNode(6);
n1.next=n2;
n2.next=n3;
System.out.println("链表2:"+n1.val+"->"+n2.val+"->"+n3.val);
ListNode result=new Solution().mergeTwoLists(m1, n1);
if(result!=null){
System.out.print("合并链表:"+result.val);
ListNode resultNext=result.next;
while(resultNext!=null){
System.out.print("->"+resultNext.val);
resultNext=resultNext.next;
}
}
}
}
class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if(l1==null) return l2;
if(l2==null) return l1;
if(l1.val<l2.val){
l1.next=mergeTwoLists(l1.next, l2);
return l1;
}else{
l2.next=mergeTwoLists(l1, l2.next);
return l2;
}
}
}
class ListNode {
int val;
ListNode next;
ListNode(int x) { val = x; }
}