21. Merge Two Sorted Lists

Problem

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

Solution (Java)

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public ListNode mergeTwoLists(ListNode l1, ListNode l2) {
        ListNode list = new ListNode(-1);
        ListNode head = list;
        while (l1 != null || l2 != null) {
            if (l1 != null && l2 != null) {
                if (l1.val <= l2.val) {
                    list.next = new ListNode(l1.val);
                    l1 = l1.next;
                } else {
                    list.next = new ListNode(l2.val);
                    l2 = l2.next;
                }
            } else if (l1 != null) {
                list.next = new ListNode(l1.val);
                l1 = l1.next;
            } else {
                list.next = new ListNode(l2.val);
                l2 = l2.next;
            }
            list = list.next;
        }
        return head.next;
    }
}

Solution (Javascript)

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
var mergeTwoLists = function(l1, l2) {
  var head = new ListNode(0);
  var now = head;
  var p1 = l1;
  var p2 = l2;
  while (p1 || p2) {
    if (p1 === null || (p2 !== null && p2.val < p1.val)) {
      now.next = p2;
      p2 = p2.next;
    } else {
      now.next = p1;
      p1 = p1.next;
    }
    now = now.next;
  }
  return head.next;
};

Explain:

nope.

Complexity:

• Time complexity : O(m+n).
• Space complexity : O(m+n).