Leetcode 0021. Merge Two Sorted Lists

You are given the heads of two sorted linked lists `list1` and `list2`. Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists. Return the head of the merged linked list.

Easy

Description

You are given the heads of two sorted linked lists list1 and list2.

Merge the two lists in a one sorted list. The list should be made by splicing together the nodes of the first two lists.

Return the head of the merged linked list.

Example 1:

Input: list1 = [1,2,4], list2 = [1,3,4]
Output: [1,1,2,3,4,4]

Example 2:

Input: list1 = [], list2 = []
Output: []

Example 3:

Input: list1 = [], list2 = [0]
Output: [0]

Constraints:

• The number of nodes in both lists is in the range [0, 50].
• -100 <= Node.val <= 100
• Both list1 and list2 are sorted in non-decreasing order.

Solution

/**
 * 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 {
  /**
   * Linked List
   * Analysis
   * Time Complexity: BigO(n)
   * Space Complexity: BigO(1)
   */
  public ListNode mergeTwoLists(ListNode list1, ListNode list2) {
    ListNode head = new ListNode();
    ListNode tail = head;
    while (list1 != null && list2 != null) {
      if (list2.val <= list1.val) {
        tail.next = list2;
        list2 = list2.next;
      }
      else {
        tail.next = list1;
        list1 = list1.next;
      }
      tail = tail.next;
    }
    tail.next = (list1 != null) ? list1 : list2;
    return head.next;
  }
}