合并两个有序链表

/**

• Definition for singly-linked list.
• struct ListNode {

• int val;
  

• ListNode *next;
  

• ListNode() : val(0), next(nullptr) {}
  

• ListNode(int x) : val(x), next(nullptr) {}
  

• ListNode(int x, ListNode *next) : val(x), next(next) {}
  

• };
  /
  class Solution {
  public:
  ListNode mergeTwoLists(ListNode* list1, ListNode* list2) {
  ListNode* list3;
  ListNode* p;
  if (list1 == nullptr && list2 == nullptr) {
  return nullptr;
  }
  if (list1 == nullptr) {
  return list2;
  }
  if (list2 == nullptr) {
  return list1;
  }
  if (list1->val <= list2->val) {
  list3 = list1;
  list1 = list1->next;
  } else {
  list3 = list2;
  list2 = list2->next;
  }
  p = list3;
  while(list1 != nullptr && list2 != nullptr) {
  if (list1->val <= list2->val) {
  p->next = list1;
  list1 = list1->next;
  p = p->next;
  } else {
  p->next = list2;
  list2 = list2->next;
  p = p->next;
  }
  }
  if (list1 != nullptr) {
  p->next = list1;
  }
  if (list2 != nullptr) {
  p->next = list2;
  }
  return list3;
  }
  };