24 反转链表

非递归实现（java）

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null){
            return head;
        }
        ListNode before = null;
        ListNode cur = head;
        ListNode memo = head.next;
        while(cur != null){
            memo = cur.next;
            cur.next = before;
            before = cur;
            cur = memo;
        }
        return before;
    }
}

递归实现（java）

public class Solution {
    public ListNode ReverseList(ListNode head) {
        if(head == null){
            return null;
        }
        ListNode pre = null;
        ListNode cur = head;
        return reverse(pre, cur);
    }
    private ListNode reverse(ListNode pre, ListNode cur){
        if(cur == null){
            return pre;
        }
        ListNode memo = cur.next;
        cur.next = pre;
        pre = cur;
        cur = memo;
        return reverse(pre, cur);
    }
}

头插法

    public ListNode ReverseList(ListNode head) {
        ListNode memo = new ListNode(-1);
        ListNode node = head;
        while (node != null) {
            ListNode newNode = new ListNode(node.val);
            newNode.next = memo.next;
            memo.next = newNode;
            node = node.next;
        }
        return memo.next;
    }