[LeetCode]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.

本题难度easy，关键在于别想复杂了。题目是有assumption的：sort都是从小到大。下面提供两个解法：

一、常规

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        //require 
        ListNode fake=new ListNode(0);
        ListNode cur=fake;
        //invariant
        while(true){
            if(l1==null){cur.next=l2;break;}
            if(l2==null){cur.next=l1;break;}
            if(l1.val>l2.val){
                cur.next=l2;
                l2=l2.next;
            }else{
                cur.next=l1;
                l1=l1.next;
            }
            cur=cur.next;
        }
        //ensure
        return fake.next;
    }
}

二、recursion
递归方法是在disuss上看到的，原文：​​​Java, 1 ms, 4 lines codes, using recursion​​​
（依然是为华人写的，再次展现中国人在算法方面的才华）

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;
        }
}