LeetCode刷题笔录 Merge Two Sorted Lists

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.

这题不能像merge sort一样用extra space。做法是new一个新的链表头作为结果链表的dummy head，将两个链表按顺序插入dummy head的链表里。

public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
ListNode p1 = l1;
ListNode p2 = l2;
ListNode fakeHead = new ListNode(0);
ListNode p = fakeHead;

while(p1 != null && p2 != null){
if(p1.val <= p2.val){
p.next = p1;
p1 = p1.next;
}
else{
p.next = p2;
p2 = p2.next;
}
p = p.next;
}
if(p1 != null)
p.next = p1;
if(p2 != null)
p.next = p2;
return fakeHead.next;

}
}