diff --git a/khj20006/202509/20 BOJ G3 Missing Number Queries.md b/khj20006/202509/20 BOJ G3 Missing Number Queries.md
new file mode 100644
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+++ b/khj20006/202509/20 BOJ G3 Missing Number Queries.md	
@@ -0,0 +1,103 @@
+```java
+import java.io.*;
+import java.util.*;
+
+class IOController {
+	BufferedReader br;
+	BufferedWriter bw;
+	StringTokenizer st;
+
+	public IOController() {
+		br = new BufferedReader(new InputStreamReader(System.in));
+		bw = new BufferedWriter(new OutputStreamWriter(System.out));
+		st = new StringTokenizer("");
+	}
+
+	String nextLine() throws Exception {
+		String line = br.readLine();
+		st = new StringTokenizer(line);
+		return line;
+	}
+
+	String nextToken() throws Exception {
+		while (!st.hasMoreTokens())
+			nextLine();
+		return st.nextToken();
+	}
+
+	int nextInt() throws Exception {
+		return Integer.parseInt(nextToken());
+	}
+
+	long nextLong() throws Exception {
+		return Long.parseLong(nextToken());
+	}
+
+	double nextDouble() throws Exception {
+		return Double.parseDouble(nextToken());
+	}
+
+	void close() throws Exception {
+		bw.flush();
+		bw.close();
+	}
+
+	void write(String content) throws Exception {
+		bw.write(content);
+	}
+
+}
+
+public class Main {
+
+	static IOController io;
+
+	//
+
+	static int N, Q;
+	static int[] cnt, a;
+	static TreeSet<Integer> set;
+
+	public static void main(String[] args) throws Exception {
+
+		io = new IOController();
+
+		N = io.nextInt();
+		Q = io.nextInt();
+		a = new int[N+1];
+		cnt = new int[N+1];
+		for(int i=1;i<=N;i++) {
+		    a[i] = io.nextInt();
+		    cnt[a[i]]++;
+		}
+		
+		set = new TreeSet<>();
+		for(int i=1;i<=N;i++) if(cnt[i] == 0) set.add(i);
+		
+		for(int i=0;i<Q;i++) {
+		    int op = io.nextInt();
+		    int x = io.nextInt();
+		    int y = io.nextInt();
+		    if(op == 1) {
+		        int prev = a[x];
+		        if(--cnt[prev] == 0) set.add(prev);
+		        if(cnt[y]++ == 0) set.remove(y);
+		        a[x] = y;
+		    }
+		    else {
+		        if(set.isEmpty()) {
+		            if(x == 1) io.write(a[r+1] + "\n");
+		            else io.write(a[1] + "\n");
+		        }
+		        else {
+		            io.write(set.first() + "\n");
+		        }
+		    }
+		}
+
+		io.close();
+
+	}
+
+}
+```