21. 合并两个有序链表

递归法

/*** 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;}if (list1->val < list2->val) {list1->next = mergeTwoLists(list1->next, list2);return list1;} else {list2->next = mergeTwoLists(list1, list2->next);return list2;}return nullptr;}
};

迭代法

/*** 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* dummy = new ListNode(-1);ListNode* node = dummy;while (list1 != nullptr && list2 != nullptr) {if (list1->val < list2->val) {node->next = list1;list1 = list1->next;} else {node->next = list2;list2 = list2->next;}node = node->next;}if (list1 == nullptr) {node->next = list2;}if (list2 == nullptr) {node->next = list1;}return dummy->next;}
};

148. 排序链表

/*** 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) {if (list1 == nullptr) {return list2;}if (list2 == nullptr) {return list1;}if (list1->val < list2->val) {list1->next = mergeTwoLists(list1->next, list2);return list1;} else {list2->next = mergeTwoLists(list1, list2->next);return list2;}return nullptr;}ListNode* sortList(ListNode* head) {if (head == nullptr || head->next == nullptr) {return head;}ListNode* fast = head->next->next;ListNode* slow = head;while (fast != nullptr && fast->next != nullptr) {fast = fast->next->next;slow = slow->next;}ListNode* right = slow->next;slow->next = nullptr;return mergeTwoLists(sortList(head), sortList(right));}
};

23. 合并 K 个升序链表

优先队列解法：

/*** 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) {}* };*/
struct compare {bool operator()(ListNode* a, ListNode* b) {return a->val > b->val;}
};
class Solution {
public:ListNode* mergeKLists(vector<ListNode*>& lists) {ListNode* dummy = new ListNode(-1);ListNode* cur = dummy;priority_queue<ListNode*, vector<ListNode*>, compare> pq;for (auto node : lists) {if (node != nullptr) {pq.push(node);}}while (!pq.empty()) {ListNode* node = pq.top();pq.pop();cur->next = node;cur = cur->next;if (node->next != nullptr) {pq.push(node->next);}}return dummy->next;}
};

递归解法：

/*** 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;}if (list1->val < list2->val) {list1->next = mergeTwoLists(list1->next, list2);return list1;} else {list2->next = mergeTwoLists(list1, list2->next);return list2;}return nullptr;}ListNode* mergeLists(vector<ListNode*>& lists, int left, int right) {if (left > right) {return nullptr;}if (left == right) {return lists[left];}int mid =  left + (right - left) / 2;return mergeTwoLists(mergeLists(lists, left, mid), mergeLists(lists, mid + 1, right));}ListNode* mergeKLists(vector<ListNode*>& lists) {return mergeLists(lists, 0, lists.size() - 1);}
};