力扣：655. 输出二叉树

/**
 * Definition for a binary tree node.
 * public class TreeNode {
 *     int val;
 *     TreeNode left;
 *     TreeNode right;
 *     TreeNode() {}
 *     TreeNode(int val) { this.val = val; }
 *     TreeNode(int val, TreeNode left, TreeNode right) {
 *         this.val = val;
 *         this.left = left;
 *         this.right = right;
 *     }
 * }
 */
class Solution {
    public List<List<String>> printTree(TreeNode root) {
        int height = height(root);
        int m = height + 1;
        int n = (1 << m) - 1;
        List<List<String>> res = new ArrayList<>();
        for (int i = 0; i < m; i++){
            List<String> temp = new ArrayList<>();
            for (int j = 0; j < n; j++){
                temp.add("");
            }
            res.add(temp);
        }
        dfs(root, res, 0, (n - 1) / 2, height);
        return res;
    }

    // 获取树的高度
    private int height(TreeNode root){
        int h = 0; 
        if (root.left != null){
            h = Math.max(h, height(root.left) + 1);
        }

        if (root.right != null){
            h = Math.max(h, height(root.right) + 1);
        }
        return h;
    }

    // 填充结果
    private void dfs(TreeNode node, List<List<String>> res, int row, int mid, int height){
        res.get(row).set(mid, Integer.toString(node.val));
        if (node.left != null){
            dfs(node.left, res, row + 1, mid - (1 << (height - row - 1)), height);
        }
        if (node.right != null){
            dfs(node.right, res, row + 1, mid + (1 << (height - row - 1)), height);
        }
    }
}