|
| 1 | +/** |
| 2 | + * Definition for singly-linked list. |
| 3 | + * struct ListNode { |
| 4 | + * int val; |
| 5 | + * ListNode *next; |
| 6 | + * ListNode() : val(0), next(nullptr) {} |
| 7 | + * ListNode(int x) : val(x), next(nullptr) {} |
| 8 | + * ListNode(int x, ListNode *next) : val(x), next(next) {} |
| 9 | + * }; |
| 10 | +*/ |
| 11 | +classSolution { |
| 12 | +public: |
| 13 | +voidreorderList(ListNode* head) { |
| 14 | + ListNode* prev =nullptr; |
| 15 | + ListNode* slow = head; |
| 16 | + ListNode* fast = head; |
| 17 | + |
| 18 | +// set slow to middle node |
| 19 | +while (fast !=nullptr && fast->next !=nullptr) { |
| 20 | + prev = slow; |
| 21 | + slow = slow->next; |
| 22 | + fast = fast->next->next; |
| 23 | + } |
| 24 | + |
| 25 | +if (prev !=nullptr) { |
| 26 | + prev->next =reverseList(slow); |
| 27 | +mergeTwoLists(head, prev->next); |
| 28 | + } |
| 29 | + } |
| 30 | + |
| 31 | +private: |
| 32 | + ListNode*reverseList(ListNode* head) { |
| 33 | + ListNode* prev =nullptr; |
| 34 | + ListNode* curr = head; |
| 35 | + |
| 36 | +while (curr !=nullptr) { |
| 37 | + ListNode* next = curr->next; |
| 38 | + curr->next = prev; |
| 39 | + prev = curr; |
| 40 | + curr = next; |
| 41 | + } |
| 42 | + |
| 43 | +return prev; |
| 44 | + } |
| 45 | + |
| 46 | +// invariant: list2 length = [list1 length, list1 length + 1] |
| 47 | + ListNode*mergeTwoLists(ListNode* list1, ListNode* list2) { |
| 48 | + ListNode dummy; |
| 49 | + ListNode* prev = &dummy; |
| 50 | + |
| 51 | + ListNode* l1 = list1; |
| 52 | + ListNode* l2 = list2; |
| 53 | + |
| 54 | +while (l1 != list2) { |
| 55 | +takeNode(prev, l1); |
| 56 | +takeNode(prev, l2); |
| 57 | + } |
| 58 | + |
| 59 | +// handle odd number of nodes |
| 60 | +if (l2 !=nullptr) { |
| 61 | +takeNode(prev, l2); |
| 62 | + } |
| 63 | + |
| 64 | +return dummy.next; |
| 65 | + } |
| 66 | + |
| 67 | +voidtakeNode(ListNode*& prev, ListNode*& curr) { |
| 68 | + prev->next = curr; |
| 69 | + prev = prev->next; |
| 70 | + curr = curr->next; |
| 71 | + } |
| 72 | +}; |