leetcode/src/s0021_merge_two_sorted_lists.cpp

#include "s0021_merge_two_sorted_lists.hpp"

ListNode* S0021::mergeTwoLists(ListNode* list1, ListNode* list2) {
  if (list1 == nullptr) {
    return list2;
  } else if (list2 == nullptr) {
    return list1;
  }
  ListNode dummy = ListNode(-std::numeric_limits<int>::max());
  if (list1->val > list2->val) {
    dummy.next = list2;
    list2 = list2->next;
  } else {
    dummy.next = list1;
    list1 = list1->next;
  }
  ListNode* current = dummy.next;
  while (!(list1 == nullptr && list2 == nullptr)) {
    if (list1 == nullptr) {
      current->next = list2;
      current = current->next;
      list2 = list2->next;
    } else if (list2 == nullptr) {
      current->next = list1;
      current = current->next;
      list1 = list1->next;
    } else {
      if (list1->val > list2->val) {
        current->next = list2;
        current = current->next;
        list2 = list2->next;
      } else {
        current->next = list1;
        current = current->next;
        list1 = list1->next;
      }
    }
  }
  return dummy.next;
}