LeetCode 206. Reverse Linked List

问题

public class Solution206 {

public static void main(String args[]){
Solution206 solution206 = new Solution206();

ListNode listNode1 = new ListNode(1);
ListNode listNode2 = new ListNode(2);
ListNode listNode3 = new ListNode(3);
ListNode listNode4 = new ListNode(4);

listNode1.next = listNode2;
listNode2.next = listNode3;
listNode3.next = listNode4;

ListNode head = solution206.reverseList(listNode1);
System.out.println(head.val);
System.out.println(head.next.val);
System.out.println(head.next.next.val);
System.out.println(head.next.next.next.val);

}

/**
* 创建一个新的链表,是倒序的
*/
public ListNode reverseList(ListNode head) {
if(head==null || head.next==null){
return head;
}

int length=0;
ListNode currentNode = head;
while(true){
if(currentNode.next!=null){
currentNode = currentNode.next;
length++;
}else{
break;
}
}

ListNode newNode = new ListNode(currentNode.val);
ListNode firstNode = newNode;
for(int i=0;i<length;i++){
int value = getNodeValue(head, length-i);
newNode.next = new ListNode(value);
newNode = newNode.next;
}

return firstNode;
}

public int getNodeValue(ListNode head, int n) {
for(int i=1;i<n;i++){
head = head.next;
}
return head.val;
}

//    /**
//     * 解析答案
//     */
//    public ListNode reverseList(ListNode head) {
//
//        ListNode pre     = null;
//        ListNode current = head;
//        while(current!=null){
//            ListNode next_temp = current.next;
//            current.next = pre;
//            pre = current;
//            current = next_temp;
//        }
//        return pre;
//    }

public static class ListNode {
int val;
ListNode next;
ListNode(int x) { val = x; }
}
}