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;

    }
}

最爲重要一點是使用了result進行初始化或叫做dummy 更貼切，這樣解決了第一個結點next 指向不必判斷是l1還是l2的結點帶來的麻煩。