BFS以及它的扩展，我发现栈是个很好用的数据结构，特别是对于顺序需要颠倒的时候！！！
这里有个重要的信息：可以用null来标识一个level的结束！！！
下面是AC代码：
1 /** 2 * Given a binary tree, return the bottom-up level order traversal of its nodes' values. 3 * (ie, from left to right, level by level from leaf to root). 4 * first BSF(but from right to left at each level), then using a stack to traverse from bottom to up 5 * @param root 6 * @return 7 */ 8 public ArrayList<ArrayList<Integer>> levelOrderBottom(TreeNode root){ 9 ArrayList<ArrayList<Integer>> r = new ArrayList<ArrayList<Integer>>(); 10 if(root == null) 11 return r; 12 // for BFS 13 LinkedList<TreeNode> queue = new LinkedList<TreeNode>(); 14 // to record the level information 15 LinkedList<Integer> level = new LinkedList<Integer>(); 16 //for down to up 17 LinkedList<Integer> stack = new LinkedList<Integer>(); 18 //to record the level information of the stack 19 LinkedList<Integer> lel = new LinkedList<Integer>(); 20 21 queue.offer(root); 22 level.offer(1); 23 while(!queue.isEmpty()){ 24 TreeNode cur = queue.poll(); 25 stack.push(cur.val);// by using stack, we can travesal from the bottom to up 26 int levelCur = level.poll(); 27 lel.push(levelCur); 28 if(cur.right!=null) 29 { 30 queue.offer(cur.right); 31 level.offer(levelCur+1); 32 } 33 if(cur.left!=null){ 34 queue.offer(cur.left); 35 level.offer(levelCur+1); 36 } 37 } 38 ArrayList<Integer> al = null; 39 int lev = -1; 40 //from stack to Arraylist 41 while(!stack.isEmpty()){ 42 int lc = lel.pop(); 43 if(lc!=lev) 44 { 45 if(al!=null) 46 r.add(al); 47 al = new ArrayList<Integer>(); 48 } 49 al.add(stack.pop()); 50 lev = lc; 51 } 52 r.add(al); 53 return r; 54 } 55 56 /** 57 * Given a binary tree, return the level order traversal of its nodes' values. (ie, from left to right, level by level). 58 * @param root 59 * @return 60 */ 61 public ArrayList<ArrayList<Integer>> levelOrder(TreeNode root){ 62 ArrayList<ArrayList<Integer>> r = new ArrayList<ArrayList<Integer>>(); 63 if(root == null) 64 return r; 65 //for BFS 66 LinkedList<TreeNode> queue = new LinkedList<TreeNode>(); 67 ArrayList<Integer> sub = new ArrayList<Integer>(); 68 69 queue.offer(root); 70 queue.offer(null);// the null node is a label for telling a new level is begin 71 while(!queue.isEmpty()){ 72 TreeNode cur = queue.poll(); 73 //to see if it is the new level begins 74 if(cur == null){ 75 //a level has finished, we have to put nodes of the level into the total result 76 r.add(sub); 77 //begin the next level 78 if(queue.isEmpty()) 79 break; 80 else 81 cur = queue.pop(); 82 queue.offer(null); 83 //to store the nodes of the new level 84 sub = new ArrayList<Integer>(); 85 sub.add(cur.val); 86 }else 87 sub.add(cur.val); 88 89 if(cur.left!=null) 90 queue.offer(cur.left); 91 if(cur.right!=null) 92 queue.offer(cur.right); 93 } 94 return r; 95 }