N1991 - 트리 순회

import java.io.IOException;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.InputStreamReader;
import java.io.OutputStreamWriter;
import java.util.StringTokenizer;
import java.util.HashMap;
import java.util.ArrayList;

public class Main {
    private static final BufferedReader BR = new BufferedReader(new InputStreamReader(System.in));
    private static final BufferedWriter BW = new BufferedWriter(new OutputStreamWriter(System.out));
    private static final HashMap<String, ArrayList<String>> TREE = new HashMap<>();
    private static final StringBuilder SB = new StringBuilder();
    private static StringTokenizer st = new StringTokenizer("");

    public static void main(String[] args) throws IOException {
        input();
        solution();
        BW.write(SB.toString());
        BW.close();
        BR.close();
    }

    private static void input() throws IOException {
        int n = Integer.parseInt(BR.readLine());
        ArrayList<String> arr;
        for(int i=0; i<n; i++) {
            st = new StringTokenizer(BR.readLine());
            arr = new ArrayList<>(2);
            String key = st.nextToken();
            arr.add(st.nextToken());
            arr.add(st.nextToken());
            TREE.put(key, arr);
        }
    }

    private static void solution() {
        preorderTraversal(TREE, "A");
        SB.append("\n");
        inorderTraversal(TREE, "A");
        SB.append("\n");
        postorderTraversal(TREE, "A");
    }

    private static void preorderTraversal(HashMap<String, ArrayList<String>> tree, String start) {
        if(".".equals(start)) return;
        SB.append(start);
        preorderTraversal(tree, tree.get(start).get(0));
        preorderTraversal(tree, tree.get(start).get(1));
    }

    private static void inorderTraversal(HashMap<String, ArrayList<String>> tree, String start) {
        if(".".equals(start)) return;
        inorderTraversal(tree, tree.get(start).get(0));
        SB.append(start);
        inorderTraversal(tree, tree.get(start).get(1));
    }

    private static void postorderTraversal(HashMap<String, ArrayList<String>> tree, String start) {
        if(".".equals(start)) return;
        postorderTraversal(tree, tree.get(start).get(0));
        postorderTraversal(tree, tree.get(start).get(1));
        SB.append(start);
    }
}