Reverse List

Head Insertion, reverse original list order

/**
 * 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 == nullptr) {
            return nullptr;
        }

        ListNode *pre = nullptr, *next = nullptr;

        while (head != nullptr) {
            next = head->next;
            head->next = pre;
            pre  = head;
            head = next;
        }

        return pre;
    }
};