C 解法, 执行用时: 4ms, 内存消耗: 1120KB, 提交时间: 2021-09-11

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

/**
 * 
 * @param head ListNode类 
 * @return  void
 */
struct ListNode* reserve(struct ListNode* head)
{
    if( !head->next)
    {
        return head;
    }
    struct ListNode* p = head->next;
    head->next = NULL;
    while(p)
    {
        struct ListNode* temp = p;
        p = p->next;
        temp->next = head->next;
        head->next = temp;
    }
    return head;
}
void reorderList(struct ListNode* head ) {
    // write code here
    if(!head || !head->next || !head->next->next)
    {
        return ;
    }
    struct ListNode* slow = head;
    struct ListNode* fast = head;
    while(fast->next)
    {
        fast = fast->next;
        if(fast->next)
        {
            slow = slow->next;
            fast = fast->next;
        }
    }
    
    slow = reserve(slow);
    struct ListNode* newList = slow->next;
    slow->next = NULL;
    fast = head;
    while(fast && newList)
    {
        struct ListNode* temp1 = newList->next;
        struct ListNode* temp2 = fast->next;
        fast->next = newList;
        newList->next = temp2;
        fast = temp2;
        newList = temp1;
    }
}

C 解法, 执行用时: 5ms, 内存消耗: 1148KB, 提交时间: 2021-09-08

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

/**
 * 
 * @param head ListNode类 
 * @return  void
 */
void reorderList(struct ListNode* head ) {
    // write code here
    struct ListNode *p = head, *q = p;
    struct ListNode *r = NULL, *s;
    if(head){
        while(q->next){

            q = q->next;
            if(q->next){
                q = q->next;
                p = p->next;
            }
            else{
                break;
            }
        }
        q = p->next;//找到第二个表
        p->next = NULL;
        p = head;
        //逆置第二个表
        while(q){
            s = q->next;
            q->next = r;
            r = q;
            q = s;
        }
        //r指向第二个表的第一个节点
        q = r;
        while(q){
            r = r->next;
            s = p->next;
            p->next = q;
            q->next = s;
            p = s;
            q = r;
        }
    }
}

C 解法, 执行用时: 5ms, 内存消耗: 1248KB, 提交时间: 2021-09-09

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

/**
 * 
 * @param head ListNode类 
 * @return  void
 */

void reorderList(struct ListNode* head) {
    if (head == NULL) {
        return;
    }
    struct ListNode* vec[40001];
    struct ListNode* node = head;
    int n = 0;
    while (node != NULL) {
        vec[n++] = node;
        node = node->next;
    }
    int i = 0, j = n - 1;
    while (i < j) {
        vec[i]->next = vec[j];
        i++;
        if (i == j) {
            break;
        }
        vec[j]->next = vec[i];
        j--;
    }
    vec[i]->next = NULL;
}

C 解法, 执行用时: 5ms, 内存消耗: 1336KB, 提交时间: 2021-11-18

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 *
 * C语言声明定义全局变量请加上static，防止重复定义
 */

/**
 * 
 * @param head ListNode类 
 * @return  void
 */
void reorderList(struct ListNode* head ) {
    // write code here
    if(head==NULL||head->next==NULL)
        return;
    struct ListNode* node =head;
    int lenth=0;
    int i=0,j=0;
    while(node!=NULL)
    {
        node=node->next;
        lenth++;
    }
    node=head;
    struct ListNode* arr[lenth];
    while(node!=NULL)
    {
        arr[i++]=node;
        node=node->next;
    }
    i=0;
    j=lenth-1;
    while(i<j)
    {
        arr[i]->next=arr[j];
        i++;
        if(i==j)
            break;
        arr[j]->next=arr[i];
        j--;
    }
    arr[i]->next=NULL;
}

C 解法, 执行用时: 5ms, 内存消耗: 1348KB, 提交时间: 2021-08-09

/**
 * struct ListNode {
 *	int val;
 *	struct ListNode *next;
 * };
 */

/**
 * 
 * @param head ListNode类 
 * @return  void
 */
void reorderList(struct ListNode* head ) {
    // write code here
    if (head == NULL) {
        return;
    }
    struct ListNode* vec[40001];
    struct ListNode* node = head;
    int n = 0;
    while (node != NULL) {
        vec[n++] = node;
        node = node->next;
    }
    int i = 0, j = n - 1;
    while (i < j) {
        vec[i]->next = vec[j];
        i++;
        if (i == j) {
            break;
        }
        vec[j]->next = vec[i];
        j--;
    }
    vec[i]->next = NULL;
}