143. Reorder List

一、题目分析

　　

　　要求不能采用复制的手段，不能使用多余的空间

　　解析：

　　可以看出规律，最后的结果是将链表右边的部分逆转之后，和左边的部分穿插合并得来：

二、代码：

　　

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    
    //逆转链表，head是下部分的链表头，last是上一个节点。
    ListNode* reverseList(ListNode* head, ListNode* last)
    {
        if(head)
        {
            ListNode* next = head->next;
            head->next = last;
            return this->reverseList(next, head);
        }
        return last;
    }
    
    int lengthOfList(ListNode* head)
    {
        int length = 0;
        while (head) {
            head = head->next;
            length++;
        }
        return length;
    }
    
    // https://leetcode.com/problems/reorder-list/description/
    void reorderList(ListNode* head) {
        
        int length = this->lengthOfList(head);

        ListNode *right = head;
        int index = length/2 - 1;
        //寻找中间节点
        while (right && index > 0) {
            right = right->next;
            index--;
        }
        
        //逆转右边的部分
        if(right)
        {
            ListNode *rightHead = right->next;
            right->next = NULL;
            right = this->reverseList(rightHead, NULL);
        }

        
        //test
        // this->dump(head);
        // this->dump(right);
        // this->dump(this->merge_list(head, right));
        this->merge_list(head, right);
    }
    
    
    ListNode* merge_list(ListNode* left, ListNode *right)
    {
        ListNode *p = new ListNode(-1);
        ListNode *head = p;
        while (left && right) {
            p->next = left;
            left = left->next;
            p = p->next;
            
            p->next = right;
            right = right->next;
            p = p->next;
        }
        
        while (left) {
            p->next = left;
            left = left->next;
            p = p->next;
        }
        
        while (right) {
            p->next = right;
            right = right->next;
            p = p->next;
        }
        
        return head->next;
    }
    
    
};