flatten 4-direction doubly linked list.

就是有上下左右四个方向的子node，每个子node也可以有上下左右四个方向的子node，要求把这玩意儿转换成正常的doubly linkedlist，O(1) space，所以不能recursive，因为有stack space。

public class MultiListToDoubleList {
	static class MultiListNode {
		int val;
		MultiListNode pre;
		MultiListNode next;
		MultiListNode up;
		MultiListNode down;
	}
	
	public static void convert(MultiListNode head) {
		if (head == null) {
			return;
		}
		
		MultiListNode tail = head;
		while (tail.next != null) {
			tail = tail.next;
		}
		
		MultiListNode cur = head;
		while (cur != null) {
			if (cur.up != null) {
				tail.next = cur.up;
				cur.up.pre = tail;
				while (tail.next != null) {
					tail = tail.next;
				}
				cur.up = null;
			}
			
			if (cur.down != null) {
				tail.next = cur.down;
				cur.down.pre = tail;
				while (tail.next != null) {
					tail = tail.next;
				}
				cur.down = null;
			}
			cur = cur.next;
		}
	}
	
}

　　

public class MultiListToDoubleList {
        static class MultiListNode {
                int val;
                MultiListNode pre;
                MultiListNode next;
                MultiListNode up;
                MultiListNode down;
        }
        . 涓€浜�-涓夊垎-鍦帮紝鐙��鍙戝竷
        public static void convert(MultiListNode head) {. visit 1point3acres.com for more.
                if (head == null) {
                        return;
                }
                
                MultiListNode tail = head;
                while (tail.next != null) {
                        tail = tail.next;
                }
                
                MultiListNode cur = head;
                while (cur != null) {
                        if (cur.up != null) {. Waral 鍗氬�鏈夋洿澶氭枃绔�,
                                tail.next = cur.up;
                                cur.up.pre = tail;
                                while (tail.next != null) {
                                        tail = tail.next;. 涓€浜�-涓夊垎-鍦帮紝鐙��鍙戝竷
                                }
                                cur.up = null;
                        }
                        . from: 1point3acres.com/bbs 
                        if (cur.down != null) {
                                tail.next = cur.down;. 鍥磋�鎴戜滑@1point 3 acres
                                cur.down.pre = tail;
                                while (tail.next != null) {. 涓€浜�-涓夊垎-鍦帮紝鐙��鍙戝竷
                                        tail = tail.next;
                                }
                                cur.down = null;
                        }
                        cur = cur.next;
                }
        }-google 1point3acres
        
}. 鐗涗汉浜戦泦,涓€浜╀笁鍒嗗湴