华为上机测试题（大数相乘-java）

public class BigData {

public static void main(String[] args) {
// TODO Auto-generated method stub
StringBuffer sb = new StringBuffer();
//        String str1 = "99999999999";
//        String str2 = "99999999999";
for (int i = 0; i < 2000; i++) {
sb.append(9);
}
String str1 = sb.toString();
String str2 = str1;
BigData test = new BigData();
String result = test.mutiStr(str1, str2);
System.out.println(result);
}

public String mutiStr(String str1, String str2) {
String result = null;// 存放结果字符串

// 确定负号
int flagStr1 = 0;
if (str1.startsWith("-")) {
flagStr1++;
}
int flagStr2 = 0;
if (str2.startsWith("-")) {
flagStr2++;
}
// *********************************************************************

// 将数字放到整数数组，逆序，字符串尾部对齐
int[] arrStr1 = new int[str1.length() - flagStr1];
for (int i = 0; i < arrStr1.length; i++) {
arrStr1[i] = str1.charAt(str1.length() - 1 - i) - '0';
}
int[] arrStr2 = new int[str2.length() - flagStr2];
for (int i = 0; i < arrStr2.length; i++) {
arrStr2[i] = str2.charAt(str2.length() - 1 - i) - '0';
}
// ***********************************************************************

// 运算开始，arrStr2的每位去乘arrStr1的每位
StringBuffer subResult = new StringBuffer();// 中间结果
for (int i = 0; i < arrStr2.length; i++) {
StringBuffer midResult = new StringBuffer();// 每位结果
int subBenWei = 0;// 每位相乘时的本位上数字
int subJinWei = 0;// 每位相乘时的进位上数字
for (int k = 0; k < i; k++) {
midResult.append(0);// 先添加0，确保每位末尾对齐，便于加法运算
}
for (int j = 0; j < arrStr1.length; j++) {
subBenWei = (arrStr2[i] * arrStr1[j] + subJinWei) % 10;
subJinWei = (arrStr2[i] * arrStr1[j] + subJinWei) / 10;
midResult.append(subBenWei);
if ((j == arrStr1.length - 1) && subJinWei > 0) {
midResult.append(subJinWei);// 如果最前面位相乘有进位，结果添加进位数字
}
}
if (0 == i) {
subResult = midResult;// 第一位，子结果存放第一位结果
} else {
// 各位对齐，确保中间结果和每位结果位数相同，便于相加运算
if (midResult.length() > subResult.length()) {
int diff = midResult.length() - subResult.length();
while (diff > 0) {
subResult.append(0);
diff--;
}
} else {
int diff = subResult.length() - midResult.length();
while (diff > 0) {
midResult.append(0);
diff--;
}
}
// *********************************************************
// 相加运算，结果放到subResult中
int benWei = 0;
int jinWei = 0;
for (int j = 0; j < subResult.length(); j++) {

benWei = ((subResult.charAt(j) - '0')
+ (midResult.charAt(j) - '0') + jinWei) % 10;
jinWei = ((subResult.charAt(j) - '0')
+ (midResult.charAt(j) - '0') + jinWei) / 10;
subResult.setCharAt(j, (char) (benWei + '0'));
}
// 如果最后相加任有进位，结果中添加进位
if (jinWei > 0) {
subResult.append(jinWei);
}
// ***********************************************************
}
}
// 判断“-”的个数，没有或有两个则为结果为正，不考虑，如只有一个则结果要添加“-”
if (1 == (flagStr1 + flagStr2)) {
subResult.append('-');
}
result = subResult.reverse().toString();// 最终子结果逆序，即为结果
return result;
}
}