leetcode 日经贴，Cpp code -Reorder List

/**
* Definition for singly-linked list.
* struct ListNode {
*     int val;
*     ListNode *next;
*     ListNode(int x) : val(x), next(NULL) {}
* };
*/
class Solution {
public:
void reorderList(ListNode *head) {
if (!head || !head->next) {
return ;
}
ListNode *l1 = head, *l2 = head, *pre = head;
while (l1 && l2) {
pre = l1;
l1 = l1->next;
if (l2->next) {
l2 = l2->next->next;
} else {
l2 = NULL;
}
}
l2 = pre->next;
pre->next = NULL;
//reverse l2
pre = l2->next;
l2->next = NULL;
while (pre) {
ListNode *next = pre->next;
pre->next = l2;
l2 = pre;
pre = next;
}
//merge
l1 = head->next;
pre = head;
while (l1 || l2) {
if (l2) {
pre->next = l2;
pre = l2;
l2 = l2->next;
}
if (l1) {
pre->next = l1;
pre = l1;
l1 = l1->next;
}
}
}
};