leetcode 143 链表对折

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode(int x) { val = x; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        ListNode p1 = head;
        ListNode p2 = head;
        while (p1 != null && p2 != null && p2.next != null) {
            p1 = p1.next;
            p2 = p2.next.next;
        }
        if (p1 != null) {
            p2 = p1.next;
            p1.next = null;
        }
        ListNode pre = reverseList(p2);
        mergeLists(head, pre);
    }
    
    public ListNode reverseList(ListNode p) {
        if (p == null)
            return p;
        ListNode pre = p;
        p = p.next;
        pre.next = null;
        while (p != null) {
            ListNode tmp = p;
            p = p.next;
            tmp.next = pre;
            pre = tmp;
        }
        return pre;
    }
    
    public void mergeLists(ListNode head, ListNode pre) {
        ListNode p1 = head;
        ListNode p2 = pre;
        ListNode tmp = null;
        while (p1 != null && p2 != null) {
            tmp = p1.next;
            p1.next = p2;
            p1 = tmp;
            tmp = p2.next;
            if (p1 == null) {
                break;
            }
            p2.next = p1;
            p2 = tmp;
        }
    }
}

面试常考的一题，比较繁琐。包含了链表中点，反转链表，合并链表这三大常考的子问题。

代码还不是最优的，后续会更新一个更清晰，简练的版本。

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