方法一 直接法 一个个比较
/**
* 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 l1, ListNode l2) {
if(l1 == null) {
return l2;
}
if(l2 == null) {
return l1;
}
ListNode head = new ListNode(0);
ListNode p = head;
while(l1 != null && l2 != null) {
if(l1.val < l2.val) {
head.next = l1;
head = head.next;
l1 = l1.next;
}else {
head.next = l2;
head = head.next;
l2 = l2.next;
}
}
while(l1 != null) {
head.next = l1;
head = head.next;
l1 = l1.next;
}
while(l2 != null) {
head.next = l2;
head = head.next;
l2 = l2.next;
}
return p.next;
}
}
方法二 递归
/**
* 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 l1, ListNode l2) {
return help(l1, l2);
}
public ListNode help(ListNode l1, ListNode l2) {
if(l1 == null) {
return l2;
}
if(l2 == null) {
return l1;
}
if(l1.val < l2.val) {
l1.next = help(l1.next, l2);
return l1;
}else {
l2.next = help(l1, l2.next);
return l2;
}
}
}