list的题最大的麻烦就在于细节，这题也是弄了挺久，思路很简单，双指针取终点，后面的list反向后搭一起就行了
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode(int x) : val(x), next(NULL) {} 7 * }; 8 */ 9 class Solution { 10 public: 11 ListNode *reverse(ListNode *head) { 12 if (!head) return NULL; 13 ListNode *pre = NULL; 14 ListNode *next; 15 next = head->next; 16 while (head) { 17 head->next = pre; 18 pre = head; 19 head = next; 20 if (next) next = next->next; 21 } 22 return pre; 23 } 24 void reorderList(ListNode *head) { 25 ListNode *p, *q; 26 p = q = head; 27 if (!head) return; 28 while (q) { 29 q = q->next; 30 if (!q) break; 31 q = q->next; 32 if (!q) break; 33 p = p->next; 34 } 35 ListNode *ppre = p; 36 p = p->next; 37 ppre->next = NULL; 38 q = reverse(p); 39 p = head; 40 ListNode *pnext, *qnext; 41 while (p) { 42 pnext = p->next; 43 if (q) qnext = q->next; 44 else break; 45 p->next = q; 46 q->next = pnext; 47 p = pnext; 48 q = qnext; 49 } 50 } 51 };
第二次一次过，用function看起来更加清楚
1 /** 2 * Definition for singly-linked list. 3 * struct ListNode { 4 * int val; 5 * ListNode *next; 6 * ListNode(int x) : val(x), next(NULL) {} 7 * }; 8 */ 9 class Solution { 10 public: 11 ListNode *reverse(ListNode *head) { 12 if (!head) return NULL; 13 ListNode *p = head; 14 ListNode *pre = NULL; 15 while (p) { 16 ListNode *pnext = p->next; 17 p->next = pre; 18 pre = p; 19 p = pnext; 20 } 21 return pre; 22 } 23 void merge(ListNode *p, ListNode *q) { 24 ListNode *head = p; 25 bool flag = true; 26 while (p && q) { 27 if (flag) { 28 ListNode *pnext = p->next; 29 p->next = q; 30 p = pnext; 31 flag = !flag; 32 } 33 else { 34 ListNode *qnext = q->next; 35 q->next = p; 36 q = qnext; 37 flag =!flag; 38 } 39 } 40 } 41 void reorderList(ListNode *head) { 42 ListNode *p = head; 43 ListNode *q = head; 44 if (!head) return; 45 while (q && q->next) { 46 q = q->next->next; 47 if (!q) break; 48 p = p->next; 49 } 50 q = p->next; 51 p->next = NULL; 52 q = reverse(q); 53 p = head; 54 merge(p, q); 55 } 56 };
C#
1 /** 2 * Definition for singly-linked list. 3 * public class ListNode { 4 * public int val; 5 * public ListNode next; 6 * public ListNode(int x) { val = x; } 7 * } 8 */ 9 public class Solution { 10 public void ReorderList(ListNode head) { 11 ListNode p = head; 12 ListNode q = head; 13 if (head == null) return; 14 while (q != null && q.next != null) { 15 q = q.next.next; 16 if (q == null) break; 17 p = p.next; 18 } 19 q = p.next; 20 p.next = null; 21 q = reverse(q); 22 p = head; 23 merge(p, q); 24 } 25 void merge(ListNode p, ListNode q) { 26 ListNode head = p; 27 bool flag = true; 28 while (p != null && q != null) { 29 if (flag) { 30 ListNode pnext = p.next; 31 p.next = q; 32 p = pnext; 33 flag = !flag; 34 } 35 else { 36 ListNode qnext = q.next; 37 q.next = p; 38 q = qnext; 39 flag = !flag; 40 } 41 } 42 } 43 public ListNode reverse(ListNode head) { 44 if (head == null) return null; 45 ListNode p = head; 46 ListNode pre = null; 47 while (p != null) { 48 ListNode pnext = p.next; 49 p.next = pre; 50 pre = p; 51 p = pnext; 52 } 53 return pre; 54 } 55 }