143. Reorder List

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}.

思路很清晰 找到中点 反转后半部分 然后reorder 

代码稍微比solution复杂点 以下是solution 

public void reorderList(ListNode head) {
    if(head==null||head.next==null) return;
    
    //Find the middle of the list
    ListNode p1=head;
    ListNode p2=head;
    while(p2.next!=null&&p2.next.next!=null){ 
        p1=p1.next;
        p2=p2.next.next;
    }
    
    //Reverse the half after middle  1->2->3->4->5->6 to 1->2->3->6->5->4
    ListNode preMiddle=p1;
    ListNode preCurrent=p1.next;
    while(preCurrent.next!=null){
        ListNode current=preCurrent.next;
        preCurrent.next=current.next;
        current.next=preMiddle.next;
        preMiddle.next=current;
    }
    
    //Start reorder one by one  1->2->3->6->5->4 to 1->6->2->5->3->4
    p1=head;
    p2=preMiddle.next;
    while(p1!=preMiddle){
        preMiddle.next=p2.next;
        p2.next=p1.next;
        p1.next=p2;
        p1=p2.next;
        p2=preMiddle.next;
    }
}