143. Reorder List學習

143. Reorder List

    Total Accepted: 71015
    Total Submissions: 301125
    Difficulty: Medium

Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→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}.

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代碼如下

package org.gsh.shop.service;

public class ReorderList {
//    class ListNode {    
//        int val;    
//        ListNode next;    
//    
//        ListNode(int x) {    
//            val = x;    
//            next = null;    
//        }    
//    }    
    
    public void reorderList(ListNode head) {    
        if (head == null || head.next == null) {    
            return;    
        }    
    
        // find the second half head    
        ListNode fast = head.next;    
        ListNode slow = head;    
        while (fast != null && fast.next != null) {    
            fast = fast.next.next;    
            slow = slow.next;    
        }    
    
        // reverse the second half    
        ListNode p = slow.next;    
        slow.next = null; // cut the first half    
        ListNode pPre = null;    
        ListNode pSuf = p.next;    
        while (p != null) {    
            pSuf = p.next;    
            p.next = pPre;    
            pPre = p;    
            p = pSuf;    
        }    
    
        // combine two halves    
        ListNode l1 = head;    
        ListNode l2 = pPre;    
        while (l1 != null && l2 != null) {    
            ListNode l1Next = l1.next;    
            ListNode l2Next = l2.next;    
            l1.next = l2;    
            l2.next = l1Next;    
            l1 = l1Next;    
            l2 = l2Next;    
        }    
    }    
}