/**
 * Definition of TreeNode:
 * public class TreeNode {
 *     public int val;
 *     public TreeNode left, right;
 *     public TreeNode(int val) {
 *         this.val = val;
 *         this.left = this.right = null;
 *     }
 * }
 */

public class Solution {
    /**
     * @param root: a TreeNode
     * @return: a list of integer
     */
    public List<Integer> boundaryOfBinaryTree(TreeNode root) {
        List<Integer> result = new ArrayList<>();

        if (root.left == null) {
            result.add(root.val);
        } else {
            getLeftSide(root, result);
            result.remove(result.size() - 1);
        }
        int leftSize = result.size();
        getBottomSide(root, result);
        if (root.right != null) {
            result.remove(result.size() - 1);
            getRightSide(root, result);
            result.remove(result.size() - 1);
        }
        return result;
    }
    private void getLeftSide(TreeNode root, List<Integer> result) {
        if (root != null) {
            result.add(root.val);
            if (root.left != null) {
                getLeftSide(root.left, result);
            } else {
                getLeftSide(root.right, result);
            }
        }
    }

    private void getBottomSide(TreeNode root, List<Integer> result) {
        if (root != null) {
            if (root.left == null && root.right == null) {
                result.add(root.val);
            } else {
                getBottomSide(root.left, result);
                getBottomSide(root.right, result);
            }
        }
    }

    private void getRightSide(TreeNode root, List<Integer> result) {
        if (root != null) {
            if (root.right != null) {
                getRightSide(root.right, result);
            } else {
                getRightSide(root.left, result);
            }
            result.add(root.val);
        }
    }
}