例： 1–>2–>3–>4–>5–>null 5–>4–>3–>2–>1–>null 思路：1 2 3 4 5 null 2 1 3 4 5 null 3 2 1 4 5 null 4 3 2 1 5 null 5 4 3 2 1 null
class Solution { public ListNode reverseList(ListNode head) { ListNode result = null; ListNode cur = head; while (cur != null) { ListNode next = cur.next; cur.next = result; result = cur; cur = next; } return result; } }or
class Solution { public ListNode reverseList(ListNode head) { if (head == null) { return null; } ListNode p1 = null; ListNode p2 = head; ListNode p3 = head.next; while (p2 != null) { p2.next = p1; p1 = p2; p2 = p3; if (p3 != null) { p3 = p3.next; } } return p1; } }