Merge Two Sorted Lists | LeetCode 21

[Written explanation coming soon]

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Here is the full implementation.

/**
 * Definition for singly-linked list.
 * function ListNode(val) {
 *     this.val = val;
 *     this.next = null;
 * }
 */
/**
 * @param {ListNode} l1
 * @param {ListNode} l2
 * @return {ListNode}
 */
let mergeTwoLists = function(l1, l2) {
  let dummy = new ListNode(-1);
  let head = dummy;

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

  if (l1 !== null) {
    dummy.next = l1;
  } else {
    dummy.next = l2;
  }

  return head.next;
};

class ListNode {
  constructor(val = null, next = null) {
    this.val = val;
    this.next = next;
  }
}