21. 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.

Example:

Input:1->2->4, 1->3->4
Output:1->1->2->3->4->4

Thoughts:

1. Recursively find the current least value between two sorted lists and link the next node of the smallest among the rest.

Code:

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* mergeTwoLists(ListNode* l1, ListNode* l2) {
        if(!l1) return l2;
        if(!l2) return l1;

        if(l1->val < l2 -> val){
            l1 -> next = mergeTwoLists(l1->next, l2);
            return l1;
        }
        else{
            l2->next = mergeTwoLists(l1, l2->next);
            return l2;
        }
    }
};

Code: (Less intuitive way, Java)

public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        if (l1 == null) return l2;
        if (l2 == null) return l1;

        ListNode head=null;
        ListNode former=null;
        while (l1!=null&&l2!=null) {
            if (l1.val>l2.val) {
                if (former==null) former=l2; else former.next=l2;
                if (head==null) head=former; else former=former.next;
                l2=l2.next;
            } else {
                if (former==null) former=l1; else former.next=l1;
                if (head==null) head=former; else former=former.next;
                l1=l1.next;
            }
        }
        if (l2!=null) l1=l2;
        former.next=l1;
        return head;

    }
};