http://rainykat.blogspot.com/2017/03/leetcode-143-reorder-listlinkedlist.html
https://leetcode.com/problems/reorder-list/#/description
1. Find the middle of list
2. Reverse the last half o list (from the next one of middle)
3. Connect 1st half and 2nd half iteratively
Complexity: Time O(n) Space O(1)
https://leetcode.com/problems/reorder-list/#/description
Given a singly linked list L: L0→L1→…→Ln-1→Ln,
reorder it to: L0→Ln→L1→Ln-1→L2→Ln-2→…
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
思路: Three stepsGiven
{1,2,3,4}
, reorder it to {1,4,2,3}
.1. Find the middle of list
2. Reverse the last half o list (from the next one of middle)
3. Connect 1st half and 2nd half iteratively
Complexity: Time O(n) Space O(1)
public void reorderList(ListNode head) { if(head == null)return; ListNode slow = head; ListNode fast = head; //1. find the middle of list while(fast != null && fast.next != null){ fast = fast.next.next; slow = slow.next; } ListNode tmp = slow.next;//2nd half start from next node of the middle one slow.next = null; //2. reverse last half of list ListNode newHead = reverseList(tmp); //3. connect 1st half to 2nd half ListNode cur = head; while(cur != null && newHead != null){ ListNode tmp1 = cur.next; ListNode tmp2 = newHead.next; cur.next = newHead; newHead.next = tmp1; cur = tmp1; newHead = tmp2; } } public ListNode reverseList(ListNode head) { if(head==null||head.next==null){ return head;} ListNode nextN = head.next; head.next = null; while(nextN != null){ ListNode tmp = nextN.next; nextN.next = head; head = nextN; nextN = tmp; } return head; }