Leetcode 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.

Analysis:

Make a dummy node, compare the two values in the two lists. If one is smaller, add it to the head. Update the head to next.

Note: the two lists may not be the same length. After all iteration, we need to check the lefts and add them to the result.

Time complexity: O(n)

Space complexity: O(1)

Code is below:

/**
 * 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) {
        // O(n)
        
        if (l1 == null) {
            return l2;
        }
        
        if (l2 == null) {
            return l1;
        }
        
        ListNode dummy = new ListNode(0);
        ListNode cur = dummy;
        
        while (l1 != null && l2 != null) {
            if (l1.val < l2.val) {
                cur.next = l1;
                l1 = l1.next;
            } else {
                cur.next = l2;
                l2 = l2.next;
            }
            
            cur = cur.next;
        }
        
        // Check if there're lefts
        if (l1 != null) {
            cur.next = l1;
        }
        if (l2 != null) {
            cur.next = l2;
        }
        
        return dummy.next;
    }
}