LeetCode: 21. Merge Two Sorted Lists

Question

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.

/**
* Definition for singly-linked list.
* public class ListNode {
*     int val;
*     ListNode next;
*     ListNode(int x) { val = x; }
* }
*/

Solution

public class Solution {
public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
if (l1 == null && l2 == null) {
return null;
}
if (l1 == null) {
return l2;
}
if (l2 == null) {
return l1;
}
ListNode result = new ListNode(0);
ListNode prev = result;
while (l1 != null && l2 != null) {
if (l1.val <= l2.val) {
prev.next = l1;
l1 = l1.next;
} else {
prev.next = l2;
l2 = l2.next;
}
prev = prev.next;
}
if (l1 != null) {
prev.next = l1;
}
if (l2 != null) {
prev.next = l2;
}
return result.next;

}
}

最为重要一点是使用了result进行初始化或叫做dummy 更贴切，这样解决了第一个结点next 指向不必判断是l1还是l2的结点带来的麻烦。