/**
 * Definition for singly-linked list.
 * struct ListNode {
 *     int val;
 *     struct ListNode *next;
 * };
 */

/* Function to reverse the linked list */
struct ListNode* reverseList(struct ListNode* head) 
{ 
    struct ListNode* prev = NULL; 
    struct ListNode* current = head; 
    struct ListNode* next = NULL; 
    while (current != NULL) { 
        // Store next 
        next = current->next; 
  
        // Reverse current node's pointer 
        current->next = prev; 
  
        // Move pointers one position ahead. 
        prev = current; 
        current = next; 
    } 
    return prev; 
} 

参考：
Reverse a linked list
leetcode - 206. Reverse Linked List