题解 | #整数与IP地址间的转换#

import java.util.*;

public class Main {

private final int N = 4;
public Main() {
}

public String convert(String str) {
    // ipv4 -> int
    if (str.contains(".")) {
        String[] fields = str.split("\\.");
        long result = 0;
        for (int i = 0; i < N; i++) {
            result = result * 256 + Integer.parseInt(fields[i]);
        }
        return "" + result;
    }
    // int -> ipv4
    else {
        long ipv4 = Long.parseLong(str);
        String result = "";
        for (int i = 0; i < N; i++) {
            result = ipv4 % 256 + "." + result;
            ipv4 /= 256;
        }
        return result.substring(0, result.length() - 1);
    }
}

public static void main(String[] args) {
    Main solution = new Main();
    Scanner in = new Scanner(System.in);
    while (in.hasNext()) {
        String str = in.next();
        String res = solution.convert(str);
        System.out.println(res);
    }
} 

}