32-打印二叉树

1. 从上往下打印出二叉树的每个节点，同层节点从左至右打印。

public class Solution {
    public ArrayList<Integer> topToBottom(TreeNode root) {
        ArrayList<Integer> list = new ArrayList<>();
        if (root == null) return list;
        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        while(!queue.isEmpty()) {
            TreeNode node = queue.poll();
            list.add(node.val);
            if (node.left != null) queue.offer(node.left);
            if (node.right != null) queue.offer(node.right);
        }
        return list;
    }
}

2.如果需要每一层分开

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new LinkedList<>();
        if (root == null) return res;
        LinkedList<TreeNode> nodes = new LinkedList<>();

        nodes.offer(root);
        while(!nodes.isEmpty()) {
            List<Integer> tmp = new LinkedList<>();
            for (int i = nodes.size();i > 0; i--) {
                TreeNode node = nodes.poll();
                tmp.add(node.val);
                if (node.left != null)  {
                    nodes.offer(node.left);
                }
                if (node.right != null)  {
                    nodes.offer(node.right);
                }
            }
            res.add(tmp);
        }
        return res;
    }
}

3.如果需要按照之字形

class Solution {
    public List<List<Integer>> levelOrder(TreeNode root) {
        List<List<Integer>> res = new ArrayList<>();
        if (root == null)   return res;

        Queue<TreeNode> queue = new LinkedList<>();
        queue.offer(root);
        boolean forward = true;

        while (!queue.isEmpty()) {
            int size = queue.size();
            List<Integer> list = new ArrayList<>();

            for (int i = 0; i < size; i++) {
                TreeNode cur = queue.poll();
                if (forward) {
                    list.add(cur.val);
                } else {
                    list.add(0, cur.val);
                }
                if (cur.left != null) queue.offer(cur.left);
                if (cur.right != null) queue.offer(cur.right);
            }
            forward = !forward;
            res.add(list);
        }

        return res;
    }
}