解析

class Solution {
public:
  ListNode *sortList(ListNode *head) {
    //TODO
  }
  ListNode *mergeList(ListNode *head1, ListNode *head2) {
    ListNode *node1 = head1;
    ListNode *node2 = head2;
    ListNode *dummy = new ListNode();
    ListNode *node = dummy;
    while (node1 != nullptr && node2 != nullptr) {
      if (node1->val <= node2->val) {
        node->next = node1;
        node = node->next;
        node1 = node1->next;
      } else {
        node->next = node2;
        node = node->next;
        node2 = node2->next;
      }
    }
    while (node1 != nullptr) {
      node->next = node1;
      node = node->next;
      node1 = node1->next;
    }
    while (node2 != nullptr) {
      node->next = node2;
      node = node->next;
      node2 = node2->next;
    }
    ListNode *res = dummy->next;
    delete dummy;
    return res;
  }
};