Merge Two Sorted Lists

Problem: Merge Two Sorted Lists

We make a new reference to list node to iterate through the whole list. The reference refers to either nodes in list 1 or list 2 and we update the reference of list 1 and list 2. Don't forget to deal with lists of different length simply by attach rest of longer lists to current reference.

Code in Python:

class Solution(object):
    def mergeTwoLists(self, l1, l2):
        """
        :type l1: ListNode
        :type l2: ListNode
        :rtype: ListNode
        """
        l = ListNode(None)
        head = l
        while l1 and l2:
            if l1.val < l2.val:
                l.next, l1 = l1, l1.next
            else:
                l.next, l2 = l2, l2.next
            l = l.next
        if l1: l.next = l1
        if l2: l.next = l2
        return head.next

Code in Java:

/**
 * 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) {
        ListNode dummy = new ListNode(0);
        ListNode l = dummy;

        while (l1 != null && l2 != null) {
            if (l1.val <= l2.val) {
                l.next = l1;
                l1 = l1.next;
            } else {
                l.next = l2;
                l2 = l2.next;
            }
            l = l.next;
        }

        if (l1 != null)
            l.next = l1;
        else
            l.next = l2;

        return dummy.next;
    }
}