0143.cpp

/**
 * 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) { return func1(head); }

  void func1(ListNode *head) {
    if (head == NULL || head->next == NULL) {
      return;
    }

    int size = 0;
    ListNode *curr = NULL;
    ListNode *last = NULL;
    ListNode *head1 = head;
    ListNode *head2 = NULL;

    // 1. Separate head1, head2
    curr = head;
    while (curr) {
      size++;
      curr = curr->next;
    }
    int half = size / 2;
    if (size % 2 == 1) {
      half += 1;
    }
    curr = head;
    last = head;
    while (half-- > 0) {
      last = curr;
      curr = curr->next;
    }
    head2 = curr;
    last->next = NULL;

    // 2. Reverse head2
    head2 = reverseList(head2);

    // 3. Intersect
    ListNode *curr1 = head1;
    ListNode *curr2 = head2;
    ListNode *next1 = NULL;
    ListNode *next2 = NULL;
    while (curr1 != NULL && curr2 != NULL) {
      next1 = curr1->next;
      next2 = curr2->next;
      curr2->next = next1;
      curr1->next = curr2;
      curr1 = next1;
      curr2 = next2;
    }

    return;
  }

  ListNode *reverseList(ListNode *head) {
    if (head == NULL || head->next == NULL) {
      return head;
    }
    ListNode *next = head->next;
    ListNode *new_head = reverseList(next);
    next->next = head;
    head->next = NULL;
    return new_head;
  }
};