Reorder List

https://oj.leetcode.com/problems/reorder-list/

Given a singly linked list L: L₀→L₁→…→L_n-1→L_n,
reorder it to: L₀→L_n→L₁→L_n-1→L₂→L_n-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}.

public void reorderList(ListNode head)

这一题其实就分三步去做。

第一步从中间分开两段，记得如果是长度为偶数则各一半，如果是奇数则左边比右边多一个。

第二步，将后半段reverse，就是普通的reverse list。

第三步，merge list。

下面就根据此把代码放上：

    public void reorderList(ListNode head) {
        if(head == null || head.next == null)
            return;
        ListNode sec_head = dividehelper(head);
        sec_head = reversehelper(sec_head);
        combinehelper(head, sec_head);
    }
    
    public ListNode dividehelper(ListNode head){
        ListNode one_step = head, two_step = head, prev = null;
        while(one_step != null && two_step != null){
            prev = one_step;
            one_step = one_step.next;
            two_step = two_step.next == null ? two_step.next : two_step.next.next;
        }
        prev.next = null;
        return one_step;
    }
    
    public ListNode reversehelper(ListNode head){
        ListNode prev = null, tmp = head, next = tmp.next;
        while(tmp != null){
            next = tmp.next;
            tmp.next = prev;
            prev = tmp;
            tmp = next;
        }
        return prev;
    }
    
    public void combinehelper(ListNode first, ListNode sec){
        while(first != null && sec != null){
            ListNode next_first = first.next;
            ListNode next_sec = sec.next;
            first.next = sec;
            sec.next = next_first;
            first = next_first;
            sec = next_sec;
        }
    }