题解 | #整数与IP地址间的转换#

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

public class Main {
    private static final int N = 4;
    public static void main(String[] args) throws IOException {
        BufferedReader reader = new BufferedReader(new InputStreamReader(System.in));
        String line = null;
        while ((line = reader.readLine()) != null) {
            System.out.println(switcher(line));
        }
    }

    private static String switcher(String ip) {
        if (ip.contains(".")) {
            // 带点的 -> 不带点的
            long result = 0L;
            String[] parts = ip.split("\\.");
            for (int i = 0; i < N; ++i) {
                int part = Integer.parseInt(parts[i]);
                result = (result << 8) + part;
            }
            return String.valueOf(result);
        } else {
            // 不带点的 -> 带点的
            StringBuilder result = new StringBuilder();
            long numip = Long.parseLong(ip);
            result.append(numip & 0xFF);
            for (int n = N; n > 1; --n) {
                numip >>= 8;
                result.insert(0, String.format("%d.", numip & 0xFF));
            }
            return result.toString();
        }
    }
}