两个大数相乘笔试题目

面试中遇到的笔试题目,手写两个大数相乘,不能用BigInteger (Java)

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/**  * @Title: Test  * @ProjectName client  * @date 2019/11/1413:53  */ public class Test {     public static void main(String[] args) {         String multiply1 = multiply("200", "200");         System.out.println(multiply1);     }     /**      * 01234  数组索引      * 400      第一次循环      *  000     第二次循环      *   000    第三次循环      * 公式：AB*CD  =  AC (BC+ AD) BD      * 67*89 = 6*8（7*8 + 6*9）7*9      * 67*89 = 48（110）63      * 63进6剩余3      * 110变成116，满十进位，进行11，剩余6，      * 48变成59。所以： 5963      * 参考博客: https://blog.csdn.net/outsanding/article/details/79472376      * @param num1      * @param num2      * @return      */     public static String multiply(String num1, String num2) {         int num1_len = num1.length();         int num2_len = num2.length();         int[] numArr = new int[num1_len + num2_len];         for (int i = 0; i < num1_len; i++) {             int a = num1.charAt(i) - '0';             for (int j = 0; j < num2_len; j++) {                 int b = num2.charAt(j) - '0';                 numArr[i + j] += a * b;             }         }         // 从个位数向前一位进位         for (int i = numArr.length - 1; i > 0; i--) {             numArr[i - 1] += numArr[i] / 10;             numArr[i] = numArr[i] % 10;         }         // 结果拼接         String ret = "";         for (int i = 0; i < numArr.length - 1; i++) {             ret += numArr[i];         }         return ret;     } }