[20260106] BOJ / G2 / 시간 여행 / 한종욱

Ukj0ng/202601/06 BOJ G2 시간 여행.md

@@ -0,0 +1,54 @@
+```
+import java.io.*;
+import java.util.Objects;
+import java.util.StringTokenizer;
+
+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 Node[] history;
+    private static int N;
+
+    public static void main(String[] args) throws IOException {
+        init();
+
+        for (int i = 1; i <= N; i++) {
+            StringTokenizer st = new StringTokenizer(br.readLine());
+            char q = st.nextToken().charAt(0);
+
+            if (q == 'a') {
+                int val = Integer.parseInt(st.nextToken());
+                history[i] = new Node(val, history[i-1]);
+            } else if (q == 's') {
+                history[i] = history[i-1].prev;
+            } else {
+                int time = Integer.parseInt(st.nextToken());
+                history[i] = history[time-1];
+            }
+
+            if (Objects.isNull(history[i])) bw.write("-1" + "\n");
+            else bw.write(history[i].val + "\n");
+        }
+        bw.flush();
+        bw.close();
+        br.close();
+    }
+
+    private static void init() throws IOException {
+        N = Integer.parseInt(br.readLine());
+        history = new Node[N + 1];
+
+        history[0] = null;
+    }
+
+    static class Node {
+        int val;
+        Node prev;
+
+        public Node(int val, Node prev) {
+            this.val = val;
+            this.prev = prev;
+        }
+    }
+}
+```