LeetCode: Reorder List

Given 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:
    void reorderList(ListNode *head) {
        // IMPORTANT: Please reset any member data you declared, as
        // the same Solution instance will be reused for each test case.
        // Get the List len.
        if (head == NULL) {
            return;
        }
        ListNode *tmp = head;
        int len = 0;
        while (tmp != NULL) {
            tmp = tmp->next;
            ++len;
        }
        int num_insertion = (len-1)/2;
        // Find the start of second half.
        tmp = head;
        int count = 0;
        while (count < len-1-num_insertion) {
            tmp = tmp->next;
            ++count;
        }
        ListNode *second_half = tmp->next;
        tmp->next= NULL;
        // Reverse the second half of the List.
        if (second_half != NULL && second_half->next != NULL) {
            ListNode * temp1 = second_half->next;
            second_half->next = NULL;
            while (temp1 != NULL) {
                ListNode *temp2 = temp1->next;
                temp1->next = second_half;
                second_half = temp1;
                temp1 = temp2;
            }
        }
        // Insert second half to first half.
        ListNode *first_half = head;
        while (second_half != NULL) {
            ListNode *temp1 = second_half->next;
            second_half->next = first_half->next;
            first_half->next = second_half;
            second_half = temp1;
            first_half = first_half->next->next;
        }
    }
    
};