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 };