ConnectFourBot.java

import com.fasterxml.jackson.databind.JsonNode;
import com.fasterxml.jackson.databind.ObjectMapper;
import java.io.*;

class ConnectFourBot {
  private static final ObjectMapper mapper = new ObjectMapper();
  
  public static void main(String[] args) {
    new ConnectFourBot().play();
  }
  
  public void play() {
    try (BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
         PrintWriter out = new PrintWriter(System.out)) {
      while (true) {
        JsonNode gameState = mapper.readTree(in.readLine());
        if (gameState.hasNonNull("winner")) {
          return;
        }
        
        int rowCount = gameState.get("board").size();
        int columnCount = gameState.get("board").get(0).size();
        
        out.println((int) Math.floor(Math.random() * columnCount));
        out.flush();
      }
    } catch (IOException e) {
      System.err.println(e.getMessage());
    }
  }
}