网上一搜一大把，搜出来的结果几乎都是我很崇敬的张孝祥老师写的这道题的思路，甚至有的直接把原文copy paste过来，没有一个用代码实现了的。于是自己琢磨了下，这里发布出来。虽然标题是一百亿，但实现结果可用于任意大整数。

直接上代码。这里只实现了大整数相加。有了这个，不难实现减、乘等其他操作。代码复制粘帖即可运行。

MyBigInteger.java

import java.util.ArrayList;import java.util.regex.Matcher;import java.util.regex.Pattern;/** * Created by Rocky on 14-3-26. */public class MyBigInteger {    private char sign = '0';   // 0 表示正数  - 表示负数    private byte[] data;    public MyBigInteger() {        this.data = "0".getBytes();    }    public MyBigInteger(String value) throws Exception {        //正则表达式，输入字符串要求以 零个或一个 - 开头，其余都是数字        Pattern pattern = Pattern.compile("^-?\d+$");        if (value == null || value.length() <= 0) {            value = "0";        }        Matcher matcher = pattern.matcher(value);        if (!matcher.find()) {            throw new Exception("the value is not a number string :" + value);        }        //获取字符串的第一个字符        char firstChar = value.charAt(0);        //data应该保存的是第一个非0数字后的字符串        if (firstChar == '-') {  //说明输入的是个负数            if (value.length() >=2) {                sign = firstChar;                value = value.substring(1);                value = getTemp(value); //得到value中第一个非0后的子字符串。            }        } else {            value = getTemp(value);        }        this.data = value.getBytes();    }    /**     * 得到一个字符串第一个非0后的字符串，如果没有找到，则返回 "0" 。如：00003435534，则返回3435534     * @return     */    private String getTemp(String value){        Pattern pattern = Pattern.compile("[^0]{1}");        Matcher matcher = pattern.matcher(value);        if (matcher.find()) {            value = value.substring(matcher.start());        } else {            value = "0";        }        return value;    }    public MyBigInteger add(MyBigInteger other) {        MyBigInteger result = new MyBigInteger();        int thisLength = this.data.length;        int otherLength = other.data.length;        int shorterLength = thisLength > otherLength ? otherLength : thisLength;        ArrayList<Byte> resultData = new ArrayList<Byte>();        int flag = 0;  //表示相加时的 进位，或相减时的 借位        int i = thisLength - 1;        int j = otherLength - 1;        int k = shorterLength;        //两个数的符号相同        if (other.sign == this.sign) {            //从两个整数的个位开始依次相加            while (k > 0) {                Integer temp = new Integer(new String(new byte[]{this.data[i]})) + new Integer(new String(new byte[]{other.data[j]})) + flag;                flag = temp / 10;  //相加结果超过10时的进位。没有超过10，进位为 0                resultData.add(0, ((temp % 10) + "").getBytes()[0]);  //把相加结果保存起来                k--;                i--;                j--;            }            //把多出的位加入到结果中            if (i == -1) {                while (j >= 0) {                    Integer temp = new Integer(new String(new byte[]{other.data[j]})) + flag;                    flag = temp / 10;                    resultData.add(0, ((temp % 10) + "").getBytes()[0]);                    j--;                }            } else if (j == -1) {                while (i >= 0) {                    Integer temp = new Integer(new String(new byte[]{this.data[i]})) + flag;                    flag = temp / 10;                    resultData.add(0, ((temp % 10) + "").getBytes()[0]);                    i--;                }            }            //最后把flag加进结果中            if (flag != 0) {                for (byte by : (flag + "").getBytes()) {                    resultData.add(0, by);                }            }            result.sign = other.sign;        } else {  //符号不同            if (thisLength > otherLength) {  //说明this表示的整数绝对值大，所以最终结果的符号为this的符号                result.sign = this.sign;                resultData = subtract(this.data, other.data);  //执行减法            } else if (thisLength < otherLength) {  //other表示的整数绝对值大，所以最终结果的符号为other的符号                result.sign = other.sign;                resultData = subtract(other.data, this.data);            } else {  //如果两个数据的位数相同                Integer thisInt = 0;                Integer otherInt = 0;                //从第一位开始比较，直到两者不相等                for (int n = 0; n < thisLength; n++) {                    thisInt = new Integer(new String(new byte[]{this.data[n]}));                    otherInt = new Integer(new String(new byte[]{other.data[n]}));                    if (!thisInt.equals(otherInt)) {   //注意这里要使用equals方法，因为这里需要比较的是两者的内容                        break;                    }                }                //如果this的绝对值大                if (thisInt > otherInt) {                    result.sign = this.sign;                    resultData = subtract(this.data, other.data);                } else {                    result.sign = other.sign;                    resultData = subtract(other.data, this.data);                }            }        }        result.data = new byte[resultData.size()];        for (int m = 0; m < resultData.size(); m++) {            result.data[m] = resultData.get(m);        }        return result;    }    private ArrayList<Byte> subtract(byte[] larger, byte[] smaller) {        ArrayList<Byte> resultData = new ArrayList<Byte>();        int flag = 0;        int i = smaller.length - 1;        int j = larger.length - 1;        int k = smaller.length;        while (k > 0) {            Integer temp = new Integer(new String(new byte[]{larger[j]})) + flag - new Integer(new String(new byte[]{smaller[i]}));            if (temp < 0) { //如果相减结果小于0，说明需要借位，则把flag置为 -1，以便下一位减去                flag = -1;                temp += 10;            } else {       //如果大于零，需要把flag置为 0.不要忘记了                flag = 0;            }            resultData.add(0, (temp + "").getBytes()[0]);            j--;            i--;            k--;        }        //下面的代码就不写注释了        while (j >= 0) {            Integer temp = new Integer(new String(new byte[]{larger[j]})) + flag;            if (temp < 0) {                flag = -1;                temp += 10;            } else {                flag = 0;            }            resultData.add(0, (temp + "").getBytes()[0]);            j--;        }        return resultData;    }    @Override    public String toString() {        String str = new String(this.data);        str = getTemp(str);        if (sign == '-' && str !="0") {            str = sign + str;        }        return str;    }}

MyBigIntegerTest.java

import junit. work.TestCase;import java.math.BigInteger;/** * Created by Rocky on 14-3-26. */public class MyBigIntegerTest extends TestCase {    public void test1() throws Exception {        String a1 = "-5453450543044355356576980545345054545453453454344435353254545345054304435535657698087756454543454345454534534543444353532545453450543044355356454543454354353450136546534534545345345054353450136546534534545345345043044355356576980657698087756454543454354353450136546534534545345345054353450136546534534545345345043044355356576980877564545434543543534501877564545434543543534501";        String b1 = "4545453453454344435353254545345054304435535657698087756454543454354345454534534543444353532545453450543044355356576980877564545434545454534534564545434543543534501365465345345453453450543534501365465345345453453450430443553565769804344435353254545345054304435535657698087756454543454354353450136546534534545345345043543534501365465345345453453450534501365465345345453453450";        MyBigInteger a = new MyBigInteger(a1);        MyBigInteger b = new MyBigInteger(b1);        MyBigInteger c = a.add(b);        System.out.println(c);        BigInteger a2 = new BigInteger(a1);        BigInteger b2 = new BigInteger(b1);        BigInteger c2 = a2.add(b2);        System.out.println(c2);        System.out.println(c2.toString().equals(c.toString()));    }}