题解 | #合法IP#

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

public class Main {
    public static void main(String[] args) {
        BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
        String a;
        StringBuffer bu = new StringBuffer();
        try {
            while ((a = in.readLine()) != null && !a.isEmpty()) {
                parse(a, bu);
            }
        } catch (IOException e) {
            throw new RuntimeException(e);
        }
        System.out.print(bu);
    }

    public static void parse(String a, StringBuffer buffer) {
        char[] charAy = a.toCharArray();
        int[] subIp = new int[4];
        int i = 0, j = 0, n = 0, cnt = 0, l = charAy.length;
        boolean illegal = false;
        while (i < l) {
            if (charAy[i] == '.') {
                if (j > 3 || cnt == 0) {//且ip的位数不能为0
                    illegal = true;
                    break;
                } else {
                    subIp[j++] = n;//j<4以防数组越界
                }
                n = 0;
                cnt = 0;
            } else if ((charAy[i] - '0' | '9' - charAy[i]) > 0) {
                if (cnt == 1 &&
                        n == 0) {//IP是数字，位数大于2，可是第一位是0，仍不是合法IP
                    illegal = true;
                    break;
                }
                n *= 10;
                n += charAy[i] - '0';
                cnt++;
                if (i == l -
                        1) {//当遍历到输入字符串最后一位时，如果j不是3，表示ip的位数不足或者过多，均不合法
                    if (j == 3) subIp[j++] = n;
                    else illegal = true;
                }
            } else {//不是数字，就不合法
                illegal = true;
                break;
            }
            i++;
        }
        if (illegal || j != 4) buffer.append("NO").append("\n");
        else {
            i = 0;
            while (i < 4) {
                if (subIp[i] > 255) {//ip中有位数大于255的，也不是合法IP
                    illegal = true;
                    break;
                }
                i++;
            }
            if (illegal) buffer.append("NO").append("\n");
            else buffer.append("YES").append("\n");
        }
    }
}