题解 | #IP地址转化#

import java.util.*;


public class Solution {
    /**
     * 代码中的类名、方法名、参数名已经指定，请勿修改，直接返回方法规定的值即可
     *
     * 
     * @param ip string字符串 
     * @return string字符串
     */
    public String IPtoNum(String ip) {
        // write code here
        StringBuffer sb = new StringBuffer("");
        String[] strs = ip.split("\\.");
        for (String str : strs) {
            sb.append(DecimalToBinary(Integer.valueOf(str)));
        }
        String tmpStr = new String(sb);
        char[] chrs = tmpStr.toCharArray();
        long ans = 0l;
        int account = 0;
        for (int i = chrs.length - 1; i > -1; i--) {
            if (Integer.valueOf(chrs[i] + "") == 1) {
                ans += ((long) Math.pow(2, account));
            }
            account++;
        }
        return String.valueOf(ans);
    }

    public String DecimalToBinary(int num) {
        StringBuffer sb = new StringBuffer("");
        while (0 != num) {
            sb.append(num % 2);
            num /= 2;
        }
        sb.reverse();
        String ans = new String(sb);
        while (ans.length() != 8) {
            ans = "0" + ans;
        }
        return ans;
    }
}