反转链表

迭代

/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     ListNode *next;
 *     ListNode(int x) : val(x), next(NULL) {}
 * };
 */
class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode* p = head;
        ListNode* q = p->next;
        ListNode* o = q->next;
        p->next = NULL;
        while(o != NULL)
        {
            q->next = p;
            p = q;
            q = o;
            o = o->next;
        }
        q->next = p;
        return q;
    }
};

递归

画图方便理解
不断返回原链表的尾节点
每次返回的过程中顺便更改节点间连接关系

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        if(head == NULL || head->next == NULL)
            return head;
        ListNode* newHead = reverseList(head->next);
        head->next->next = head;
        head->next = NULL;
        return newHead;
    }
};