LeetCode --- Partition List

题目链接

又是一个考察对链表基本操作的题目

附上代码：

 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 *partition(ListNode *head, int x) { 12         if (head == NULL || head->next == NULL) { 13             return head; 14  } 15         // "P" holds the track of the linked list 16         // "pre" pointer to the previous node of "p"

17         ListNode *p = head, *pre = head, *last = head; 18         // "length" holds the length of linked list

19         int length = 0; 20         while (last->next != NULL) { 21             last = last->next; 22             length++; 23  } 24         length++; 25         while (p != NULL && length--) { 26             // if "p->val" is greater than or equal to x

27             if (p->val >= x) { 28                 ListNode *q = p; 29                 // if "q" is the first node

30                 if (q == head) { 31                     head = q->next; 32                     p = head; 33                 } else if (q == last) { // if "q" is the last node

34                     break; 35                 } else { // otherwise

36                     pre->next = q->next; 37                     p = pre->next; 38  } 39                 // update "last"

40                 last->next = q; 41                 q->next = NULL; 42                 last = q; 43             } else { 44                 pre = p; 45                 p = p->next; 46  } 47  } 48         

49         return head; 50  } 51 };