Prove safety offer: reverse list (recursively, leetcode206)

topic:

 

After entering a list inverted list, the new list of the output header.

answer:

Solution one:

The first idea is one of a deposit after the next, as follows prior to use pre deposit:

/*
public class ListNode {
    int val;
    ListNode next = null;

    ListNode(int val) {
        this.val = val;
    }
}*/
public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null||head.next==null){
            return head;
        }
        ListNode pre = null;
        ListNode next = null;
        while(head!=null){
            next = head.next;
            head.next = pre;
            pre = head;
            head = next;
        }
        return pre;
    }
}

Solution two:

Recursively, as follows:

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head==null||head.next==null){
            return head;
        }
        ListNode node = ReverseList(head.next);
        head.next.next = head;
        head.next = null;
        return node;
    }
}