leetcode Reorder List

Reorder List

Total Accepted: 2069 Total Submissions: 11880My SubmissionsGiven a singly linked list L: L0→L1→…→Ln-1→Ln,reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…You must do this in-place without altering the nodes' values.For example,Given

{1,2,3,4}

, reorder it to

{1,4,2,3}

.

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
 public:
  ListNode* reverse(ListNode* head) {
    ListNode* prev = NULL, *cur = head, *next = NULL;
    while (cur != NULL) {
      next = cur->next;
      cur->next = prev;
      prev = cur;
      cur = next;
    }
    return prev;
  }
  void reorderList(ListNode *head) {
    if (head == NULL || head->next == NULL || head->next->next == NULL)
      return;
    ListNode *slow = head, *fast = head;
    while (fast != NULL && fast->next != NULL && fast->next->next != NULL) {
      slow = slow->next;
      fast = fast->next->next;
    }
    ListNode *l2 = reverse(slow->next), *l1 = head, *l1next = NULL, *l2next = NULL;
    slow->next = NULL;
    while (l2 != NULL) {
      l1next = l1->next;
      l1->next = l2;
      l2next = l2->next;
      l2->next = l1next;
      l1 = l1next;
      l2 = l2next;
    }
  }
  
};

Reverse the second half string.