BOJ 5639. 이진 검색 트리

import java.io.BufferedReader;
import java.io.InputStreamReader;

/**
 * Created by iyoungman on 2020-04-28.
 */

public class Main {

    public static void main(String[] args) throws Exception {
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        TreeNode root = new TreeNode(Integer.parseInt(br.readLine()));//(1)

        String line = "";
        while (true) {
            line = br.readLine();
            if (line == null || line.length() == 0) {
                break;
            }

            //(2)
            root = root.add(root, Integer.parseInt(line));
        }

        postOrder(root);
    }

    //(3)
    private static void postOrder(TreeNode root) {
        if (root.left != null) {
            postOrder(root.left);
        }

        if (root.right != null) {
            postOrder(root.right);
        }

        System.out.println(root.data);
    }
}

class TreeNode {

    int data;
    TreeNode left;
    TreeNode right;

    public TreeNode(int data) {
        this.data = data;
    }

    public TreeNode add(TreeNode currentNode, int data) {
        if (currentNode == null) {
            return new TreeNode(data);
        }

        if (currentNode.data > data) {
            currentNode.left = add(currentNode.left, data);//왼쪽 트리에 data 추가
        } else if (currentNode.data < data) {
            currentNode.right = add(currentNode.right, data);//오른쪽 트리에 data 추가
        }

        return currentNode;
    }
}