206. Reverse Linked List

Reverse a linked list

Thoughts:

1. Using Stack (time limit exceed)

2. Recursively: Find the second from the last node (if it Linked List contains more than two nodes), then reverse current two nodes by first creating a cycle, then cut the forward links (set it to be NULL/nullptr)

3. Having three pointers

   1. first record next node and then set current node next to pre

   2. repeat for the next step: move pre to head and then move head to next

4. 1. having three pointers with dummy node:

   2. pre always inserts cur->next right next to itself

Code 2

/**
 * 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 || !head->next) return head;
        ListNode* node = reverseList(head->next);
        head->next->next = head;
        head->next = nullptr;

        return node;
    }
};

Code 3

/**
 * 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) {
        ListNode * pre = nullptr;
        while(head){
            ListNode *next = head->next;
            head->next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
};

Code 4

class Solution {
public:
    ListNode* reverseList(ListNode* head) {
        ListNode* pre = new ListNode(0);
        pre -> next = head;
        ListNode* cur = head; 
        while (cur && cur -> next) {
            ListNode* temp = pre -> next;
            pre -> next = cur -> next;
            cur -> next = cur -> next -> next; 
            pre -> next -> next = temp;
        }
        return pre -> next;
    }
};

Special Thanks to jianchaolifighter's Solution and and redace85's Solution