143:Reorder List【链表】

题目链接：https://leetcode.com/problems/reorder-list/

/*题意：给出链表L: L0→L1→…→Ln-1→Ln
        转换成 L0→Ln→L1→Ln-1→L2→Ln-2→…
 */

/**
 *思路：将链表分成两段，分别为A和B，将B反转后插入A中
 */
class Solution {
public:
    void show(ListNode *head) {
        while(head != NULL) {
            cout << head->val << " ";
            head = head->next;
        }
        cout << endl;
    }
    int getListLength(ListNode *head) {
        int res = 1;
        while(head->next != NULL) {
            res ++;
            head = head->next;
        }
        return res;
    }
    //将链表分成两段A和B，返回B的头结点
    //B的长度大于等于A的长度
    ListNode* getRightListHead(ListNode *head, int k) {
        for(int i = 1; i < k; i ++)
            head = head->next;
        ListNode *res = head->next;
        head->next = NULL;
        return res;
    }
    //反转链表
    void RotateList(ListNode *&head) {
        ListNode *root = new ListNode(0);

        ListNode *cur = head;
        while(cur != NULL) {
            ListNode *next = cur->next;
            cur->next = root->next;
            root->next = cur;
            cur = next;
        }
        head = root->next;
    }
    void InsertList(ListNode *&headA, ListNode *&headB) {
        ListNode *curA = headA;
        ListNode *curB = headB;
        while(curA->next != NULL) {
            ListNode *nextA = curA->next;
            ListNode *nextB = curB->next;
            curA->next = curB;
            curB->next = nextA;
            curA = nextA;
            curB = nextB;
        }
        curA->next = curB;
    }
    void reorderList(ListNode* head) {
        if(head == NULL) return;
        int Length = getListLength(head);
        if(Length <= 2) return;
        ListNode *headA = head;
        ListNode *headB = getRightListHead(head, Length>>1);
        RotateList(headB);
        InsertList(headA, headB);
        head = headA;
    }
};