按层打印二叉树以及按之字形打印二叉树

定义树节点

public static class TreeNode {
		int value;
		TreeNode left;
		TreeNode right;

		public TreeNode(int val) {
			this.value = val;
		}
	}

按层打印

	// 按层 用队列
	public static void printByLayer(TreeNode root) {
		Queue<TreeNode> queue = new LinkedList<>();
		if (root == null) {
			return;
		}

		queue.offer(root);
		while (queue.size() != 0) {
			TreeNode cur = queue.poll();
			System.out.println(cur.value);
			if (cur.left != null) {
				queue.offer(cur.left);
			}
			if (cur.right != null) {
				queue.offer(cur.right);
			}
		}
	}

按之字打印

public static ArrayList<ArrayList<Integer>> printLikeZhi(TreeNode root) {
		ArrayList<ArrayList<Integer>>  res = new ArrayList<>();
		// 用两个栈 接收奇数层和偶数层
		Stack<TreeNode> s1 = new Stack<>(); // 奇数层
		Stack<TreeNode> s2 = new Stack<>(); // 偶数层		
		ArrayList<Integer> list = new ArrayList<>();
		if (root == null) {
			return res;
		}
		s1.push(root);
		while (!s1.isEmpty() || !s2.isEmpty()) {
			while (!s1.isEmpty()) { // 奇数层不为空
				TreeNode cur = s1.pop(); // 弹出s1
				// System.out.println(cur.value);
				list.add(cur.value);
				if (cur.left != null) { // s2中 加入从s1弹出的孩子 先加左再加右
					s2.push(cur.left);
				}
				if (cur.right != null) {
					s2.push(cur.right);
				}
			}
			if (list.size() > 0) {
				res.add(new ArrayList<>(list));			
				list.clear();
			}
			while (!s2.isEmpty()) { // 偶数层不为空
				TreeNode cur = s2.pop(); // 弹出s1
				list.add(cur.value);
				// System.out.println(cur.value);
				if (cur.right != null) { // s1中 加入从s2弹出的孩子 先加右再加左
					s1.push(cur.right);
				}
				if (cur.left != null) {
					s1.push(cur.left);
				}

			 }
			if (list.size() > 0) {
				res.add(new ArrayList<>(list));				
				list.clear();
			}
		}
		return res;
	}