2026/02/25

[Leetcode] 21 Merge Two Sorted Lists

 寫得好冗= ="

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/**
 * 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) {
        if (list1 == nullptr)
          return list2;
        if (list2 == nullptr)
          return list1;

        ListNode* curNode = nullptr;
        if (list1->val <= list2->val) {
          curNode = list1;
          list1 = list1->next;
        }
        else {
          curNode = list2;
          list2 = list2->next;
        }
        ListNode* startNode = curNode;

        while (list1 != nullptr && list2 != nullptr) {
          if (list1->val <= list2->val) {
            curNode->next = list1;
            curNode = list1;
            list1 = list1->next;
          }
          else {
            curNode->next = list2;
            curNode = list2;
            list2 = list2->next;
          }
        }

        if (list1 != nullptr && list2 == nullptr)
          curNode->next = list1;
        else if (list1 == nullptr && list2 != nullptr)
          curNode->next = list2;

        return startNode;
    }
};

改良 : 加個dummy

  
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/**
 * 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* startNode = new ListNode(5566);
    ListNode* curNode = startNode;

    while (list1 != nullptr && list2 != nullptr) {
      if (list1->val <= list2->val) {
        curNode->next = list1;
        list1 = list1->next;
      }
      else {
        curNode->next = list2;
        list2 = list2->next;
      }
      curNode = curNode->next;
    }

    if (list1 != nullptr && list2 == nullptr)
      curNode->next = list1;
    else if (list1 == nullptr && list2 != nullptr)
      curNode->next = list2;

    return startNode->next;
  }
};
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