[Merge Sort] 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.

一、比较排序

/** * Definition for singly-linked list. * public class ListNode { *     int val; *     ListNode next; *     ListNode(int x) { val = x; } * } */public class Solution {    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {        if (l1 == null) {            return l2;        }        if (l2 == null) {            return l1;        }        //It is a good habit that we do not  revise the parameters passed from outside.        ListNode p1 = l1;        ListNode p2 = l2;                ListNode fakeHead = new ListNode(0);        ListNode cursor = fakeHead;                while (p1 != null && p2 != null) {            if (p1.val < p2.val) {                cursor.next = p1;                p1 = p1.next;            } else {                cursor.next = p2;                p2 = p2.next;            }            cursor = cursor.next;        }        if (p1 != null) {            cursor.next = p1;        } else if (p2 != null) {            cursor.next = p2;        }                return fakeHead.next;            }}

二、递归求解

public class Solution {    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {        if (l1 == null) {            return l2;        }        if (l2 == null) {            return l1;        }        if (l1.val < l2.val) {            l1.next = mergeTwoLists(l1.next, l2);            return l1;        } else {            l2.next = mergeTwoLists(l1, l2.next);            return l2;        }    }}