reorderList.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) {
        if (!head || !head->next) return;

        // find the middle node: O(n)
        ListNode *p1 = head, *p2 = head->next;
        while (p2 && p2->next) {
            p1 = p1->next;
            p2 = p2->next->next;
        }

        // cut from the middle and reverse the second half: O(n)
        ListNode *head2 = p1->next;
        p1->next = NULL;

        p2 = head2->next;
        head2->next = NULL;
        while (p2) {
            p1 = p2->next;
            p2->next = head2;
            head2 = p2;
            p2 = p1;
        }

        // merge two lists: O(n)
        for (p1 = head, p2 = head2; p1; ) {
            auto t = p1->next;
            p1 = p1->next = p2;
            p2 = t;
        }
    }
};