lintcode35- Reverse Linked List- easy

Reverse a linked list.

For linked list 1->2->3, the reversed linked list is 3->2->1

Reverse it in-place and in one-pass 

 

用两个指针可以做到。这种链接的在纸上先用图形+语句画好很有帮助。

 

/**
 * Definition for ListNode.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int val) {
 *         this.val = val;
 *         this.next = null;
 *     }
 * }
 */


public class Solution {
    /*
     * @param head: n
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        // write your code here
        if (head == null){
            return null;
        }
        
        if (head.next == null){
            return head;
        }
        
        ListNode first = head;
        ListNode second = head.next;
        ListNode res = null;
        
        first.next = null;
        
        while(second != null){
            ListNode temp = second.next;
            second.next = first;
            first = second;
            second = temp;
        }
        
        return first;
    }
}

网上看到的精简实现版：

直接免去了我额外创新指针，head也拿来用，且巧妙自我化解开头的非法输入问题。

public class Solution {
    /**
     * @param head: The head of linked list.
     * @return: The new head of reversed linked list.
     */
    public ListNode reverse(ListNode head) {
        ListNode prev = null;
        while (head != null) {
            ListNode temp = head.next;
            head.next = prev;
            prev = head;
            head = temp;
        }
        return prev;
    }
}