21 +java + 链表合并

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

case corner
如果list1 为null 直接返回 list2
如果list2 为 null 直接返回 list1

创建一个新的链表
头为 dummy
使用pre指向该节点
循环 比较 list 1 和 list2
那个值小，就把 它连接到 新的链表上
直到其中一个list已经 为空了
然后，终止循环。

将该list后面的元素直接 连接到 pre所在的新链表后面。

class Solution {
    public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
        
        if(list1 == null) return list2;
        if(list2 == null) return list1;
        
        ListNode dummy = new ListNode(0);
        ListNode pre = dummy;
        
        while (list1 != null && list2 != null){
            if (list1.val <= list2.val){
                pre.next = list1;
                list1 = list1.next;
            } else{
                pre.next = list2;
                list2 = list2.next;
            }
            pre = pre.next;
        }
        
        if (list1 == null){
            pre.next = list2;
        } else{
            pre.next = list1;
        }
        
        return dummy.next;
    }
    

}

https://leetcode.com/problems/merge-two-sorted-lists/