合并递增链表

Question

构造两个以整型数字为值的链表，其中的值是单调递增的。 将两个链表合并，保持递增。 要求空间复杂度 O(1)

function ListNode(val) {
  this.val = val;
  this.next = null;
}

function mergeTwoLists(l1, l2) {
  if (!l1) return l2;
  if (!l2) return l1;
  let dummy = new ListNode(-1);
  let curr = dummy;
  while (l1 && l2) {
    if (l1.val <= l2.val) {
      curr.next = l1;
      l1 = l1.next;
    } else {
      curr.next = l2;
      l2 = l2.next;
    }
    curr = curr.next;
  }
  curr.next = l1 ? l1 : l2;
  return dummy.next;
}

// Example usage:
let l1 = new ListNode(1);
l1.next = new ListNode(3);
l1.next.next = new ListNode(5);

let l2 = new ListNode(2);
l2.next = new ListNode(4);
l2.next.next = new ListNode(6);

let merged = mergeTwoLists(l1, l2);

while (merged) {
  console.log(merged.val);
  merged = merged.next;
}
// Out