• 本篇來源于前段時(shí)間網(wǎng)申筆試時(shí)的一道編程題，其題意就是不調(diào)用已有的 API ，自己設(shè)計(jì)數(shù)據(jù)結(jié)構(gòu)實(shí)現(xiàn)大整數(shù)的封裝以及加法、乘法運(yùn)算。
• 題主主學(xué) JAVA 知識(shí)，當(dāng)然知道 JAVA 提供了 BigInteger 和 BigDecimal 兩個(gè)類來實(shí)現(xiàn)大整數(shù)的加、減、乘、除等運(yùn)算，兩個(gè)類都位于 java.math 包下。項(xiàng)目中若需要直接調(diào)用即可。但是針對(duì)這道筆試題，我當(dāng)時(shí)也認(rèn)真思考下，大概就是利用數(shù)組實(shí)現(xiàn)。我就直接上代碼，代碼中有注釋詳解。
• 值得閱讀者注意的就是，我代碼中是怎么將序列翻轉(zhuǎn)，再翻轉(zhuǎn)分別存儲(chǔ)在數(shù)組中，且為啥從數(shù)組最后一個(gè)元素開始輸出。還有就是代碼中引入了第三方的包 org.apache.commons.lang3.StringUtils下載地址為

import org.apache.commons.lang3.StringUtils;

public class BigInt {

    public static void main(String[] args) {
        // TODO Auto-generated method stub
        
        BigInt bigInt1 = new BigInt("1231723333333131343453458938458937834234242534532");
        BigInt bigInt2 = new BigInt("234242342342387492746278462783492748926742342424");
        System.out.println(bigInt1.toString());
        System.out.println(bigInt2.toString());
        System.out.println(bigInt1.addBigInt(bigInt2));
        System.out.println(bigInt1.multiply(bigInt2));
        /*System.out.println(bigInt1.subtract(bigInt2));*/
    
    }
    public BigInt(String bigbigNum) {
        splitToArray(bigbigNum);
    }
    
    private int[] arrayInt = new int[100];
    
    //強(qiáng)數(shù)據(jù)拆分為數(shù)組
    public void splitToArray(String bigNum){
        StringBuffer sb = new StringBuffer();
        //先將 字符串逆序
        bigNum = new StringBuffer(bigNum).reverse().toString();
        int j = 0;
        for (int i = 0; i < bigNum.length(); i++) {
            // 用一個(gè)整形數(shù)組來儲(chǔ)存，且長度不超過 4位
            if (i%4 ==0) {
                //再講sb逆序之后 存儲(chǔ)于整形數(shù)組第i和元素中
                if(sb !=null && !sb.toString().equals("")){
                    arrayInt[j] = Integer.valueOf(sb.reverse().toString());
                    sb = new StringBuffer();
                    j++;
                }
            }
            sb.append(bigNum.charAt(i));
        }
        //處理最后幾個(gè)不滿4位的數(shù)據(jù)
        if( sb!=null ){
            arrayInt[j] = Integer.valueOf(sb.reverse().toString());
        }
    }
    
    //從數(shù)組后開始打印輸出，才能得到原序列
    public String printArrayInt(int[] array){
        String str = "";
    
        StringBuffer sb = new StringBuffer();
        boolean isFirstArrayInt = true;
        for (int i = array.length-1; i >=0; i--) {
            if (array[i] != 0) {
                System.out.println("第"+i+"個(gè)數(shù)據(jù):"+array[i]);
                //判斷每個(gè)數(shù)組元素是否都為 4位，不足4位在左邊補(bǔ)0
                if (!isFirstArrayInt && String.valueOf(array[i]).length()<4) {
                    sb.append(StringUtils.leftPad(String.valueOf(array[i]),4,"0"));
                }else {
                    sb.append(array[i]);
                    if(isFirstArrayInt)
                        isFirstArrayInt = false;
                }
            }
        }
        return sb.toString();
    }
    
    //利用數(shù)組實(shí)現(xiàn)大整數(shù)的加法
    public String addBigInt(BigInt bigInt){
        int a[] = this.arrayInt;
        int b[] = bigInt.arrayInt;
        int result[] = new int[100];
        for (int i = 0; i < a.length; i++) {
            if (a[i] == 0 && b[i] != 0) {
                result[i] = b[i];
            }else if (a[i] != 0 && b[i] == 0) {
                result = a;
            }else if (a[i] != 0 && b[i] != 0) {
                result[i] = a[i]+b[i];
            }else {
                result[i] = 0;
            }
        }
        //處理result的進(jìn)位
        for (int i = 0; i < result.length; i++) {
            if (result[i] != 0 && String.valueOf(result[i]).length()>4) {
                result[i] = Integer.valueOf(String.valueOf(result[i]).substring(1));
                result[i+1] = result[i+1] +1;
            }
        }
        return printArrayInt(result);
    }
    //實(shí)現(xiàn)大整數(shù)乘法
    public String multiply(BigInt bigInt) {
        int a[] = this.arrayInt;
        int b[] = bigInt.arrayInt;
        int result[] = new int[100];
        for (int i = 0; i < a.length; i++) {
            if (a[i] == 0 || b[i] == 0) {
                result[i] = 0;
            }else {
                result[i] = a[i] * b[i];
            }
        }
        //處理result的進(jìn)位
        for (int i = 0; i < result.length; i++) {
            if (result[i] != 0 && String.valueOf(result[i]).length()>4) {
                result[i] = Integer.valueOf(String.valueOf(result[i]).substring(1));
                result[i+1] = result[i+1] +1;
            }
        }
        return printArrayInt(result);
    }
    @Override
    public String toString(){
        return printArrayInt(arrayInt);
    }
}