題目

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}.

方法

    public void reorderList(ListNode head) {
    	
    	if (!(head == null || head.next == null || head.next.next == null)) {
        	int len = 0;
        	ListNode cur = head;
        	while(cur != null) {
        		len++;
        		cur = cur.next;
        	}
        	//System.out.println("len:" + len);
        	int half = (len + 1) / 2;
        	cur = head;
        	ListNode second = head.next;
        	for (int i = 1; i < half; i++) {
        		cur = second;
        		second = second.next;
        	}
        	cur.next = null;
        	ListNode secondHead;
        	
        	cur = second.next;
        	secondHead = second;
        	secondHead.next = null;
        	while (cur != null) {
        		ListNode temp = cur;
        		cur = cur.next;
        		temp.next = secondHead;
        		secondHead = temp;
        		
        	}
        	
        	ListNode cur1 = head;
        	ListNode cur2 = secondHead;
        	while(cur2 != null) {
        		ListNode next1 = cur1.next;
        		ListNode next2 = cur2.next;
        		cur2.next = cur1.next;
        		cur1.next = cur2;
        		cur1 = next1;
        		cur2 = next2;
        	}
       	
    	}

    }