Merge Two Sorted Lists

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.

Time complexity: O(m+n)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) {
 *         val = x;
 *         next = null;
 *     }
 * }
 */
public class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        // Start typing your Java solution below
        // DO NOT write main() function
        ListNode root = null;
        ListNode node = null;
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                if (node == null) {
                    node = l1;
                    root = node;
                }
                else {
                    node.next = l1;
                    node = node.next;
                }
                l1 = l1.next;
            }
            else {
                if (node == null) {
                    node = l2;
                    root = node;
                }
                else {
                    node.next = l2;
                    node = node.next;
                }
                l2 = l2.next;
            }
        }
        
        while (l1 != null) {
            if (node == null) {
                node = l1;
                root = node;
            }
            else {
                node.next = l1;
                node = node.next;
            }
            l1 = l1.next;
        }
        
        
        while (l2 != null) {
            if (node == null) {
                node = l2;
                root = node;
            }
            else {
                node.next = l2;
                node = node.next;
            }
            l2 = l2.next;
        }
        return root;
    }
}