[LeetCode 解題報告]143. 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 may not modify the values in the list's nodes, only nodes itself may be changed.

Example 1:

Given 1->2->3->4, reorder it to 1->4->2->3.

Example 2:

Given 1->2->3->4->5, reorder it to 1->5->2->4->3.

考察：單鏈表，主要涉及快慢指針，單鏈表逆置，合併兩鏈表；

Ex 1:

step1: head: 1->2  last: 3->4;

step2: head: 1->2  pre: 4->3;

step3: 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) {
        if (head == NULL || head->next == NULL || head->next->next == NULL)
            return ;
        
        ListNode *fast, *slow;
        fast = head, slow = head;
        while (fast->next && fast->next->next) {
            fast = fast->next->next;
            slow = slow->next;
        }
        
        ListNode *mid = slow->next;
        slow->next = NULL;
        ListNode *last = mid;
        
        ListNode *pre = NULL;
        while (last) {
            ListNode *tmp = last->next;
            last->next = pre;
            pre = last;
            last = tmp;
        }
        
        while(head && pre) {
            ListNode *tmp = head->next;
            head->next = pre;
            pre = pre->next;
            head->next->next = tmp;
            head = tmp;
        }
    }
};