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  • 143. Reorder List

    Given a singly linked list LL0→L1→…→Ln-1→Ln,
    reorder it to: L0→LnL1→Ln-1→L2→Ln-2→…

    You must do this in-place without altering the nodes' values.

    For example,
    Given {1,2,3,4}, reorder it to {1,4,2,3}.

    1.找到中点

    2.reverse后半段 注意 reverse(slow.next) 并且 slow.next = null;

    3.merge

    /**
     * Definition for singly-linked list.
     * public class ListNode {
     *     int val;
     *     ListNode next;
     *     ListNode(int x) { val = x; }
     * }
     */
    public class Solution {
        public void reorderList(ListNode head) {
            if(head == null || head.next == null || head.next.next == null) return;
            ListNode slow = head;
            ListNode fast = head;
            while(fast != null && fast.next != null){
                slow = slow.next;
                fast = fast.next.next;
            }
            fast = reserve(slow.next);
            slow.next=null;
            slow = head;
            while(fast != null){
                ListNode temp = slow.next;
                ListNode temp2 = fast.next;
                slow.next = fast;
                slow = temp;
                fast.next = temp;
                fast = temp2;
            }
        }
        
        public ListNode reserve(ListNode head){
            if(head == null || head.next == null) return head;
            ListNode dummy = new ListNode(0);
            dummy.next = head;
            ListNode pre = dummy;
            ListNode first = head;
            ListNode cur = head.next;
            while(cur != null){
                first.next = cur.next;
                cur.next = pre.next;
                pre.next = cur;
                cur= first.next;
            }
            return pre.next;
        }
    }
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  • 原文地址:https://www.cnblogs.com/joannacode/p/6121563.html
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