143. 重排链表.java

/**
 * Definition for singly-linked list.
 * public class ListNode {
 *     int val;
 *     ListNode next;
 *     ListNode() {}
 *     ListNode(int val) { this.val = val; }
 *     ListNode(int val, ListNode next) { this.val = val; this.next = next; }
 * }
 */
class Solution {
    public void reorderList(ListNode head) {
        HashMap<Integer,ListNode> a=new HashMap();
        if(head==null) return;
        int index=0;
        while(head!=null){
            a.put(index,head);
            index++;
            head=head.next;
        }
        ListNode b=a.get(0);
        int i=1;
        while(i<=index/2){
            b.next=a.get(index-i);
            b=b.next;
            b.next=a.get(i);
            i++;
            b=b.next;
        }
        b.next=null;

    }
}






class Solution {
    public void reorderList(ListNode head) {
        if (head == null) {
            return;
        }
        ListNode mid = middleNode(head);
        ListNode l1 = head;
        ListNode l2 = mid.next;
        mid.next = null;
        l2 = reverseList(l2);
        mergeList(l1, l2);
    }

    public ListNode middleNode(ListNode head) {
        ListNode slow = head;
        ListNode fast = head;
        while (fast.next != null && fast.next.next != null) {
            slow = slow.next;
            fast = fast.next.next;
        }
        return slow;
    }

    public ListNode reverseList(ListNode head) {
        ListNode prev = null;
        ListNode curr = head;
        while (curr != null) {
            ListNode nextTemp = curr.next;
            curr.next = prev;
            prev = curr;
            curr = nextTemp;
        }
        return prev;
    }

    public void mergeList(ListNode l1, ListNode l2) {
        ListNode l1_tmp;
        ListNode l2_tmp;
        while (l1 != null && l2 != null) {
            l1_tmp = l1.next;
            l2_tmp = l2.next;

            l1.next = l2;
            l1 = l1_tmp;

            l2.next = l1;
            l2 = l2_tmp;
        }
    }
}

作者：LeetCode-Solution
链接：https://leetcode-cn.com/problems/reorder-list/solution/zhong-pai-lian-biao-by-leetcode-solution/
来源：力扣（LeetCode）
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