实现两个大数相乘

牛客网链接-大数相乘

思路：

1. 内心知道这种方法是可以通过的，但是贼鸡儿麻烦。
2. 输入两个字符串。按照笔算乘法的方式，循环计算。考虑到进位的问题。

代码：

package top.simuhunluo;

import java.util.Scanner;

public class Main {
    static StringBuffer[] results;
    static boolean z = true;
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        String a = scanner.next();
        String b = scanner.next();
        int a_len = a.length();
        int b_len = b.length();
        StringBuffer str_a = new StringBuffer(a);
        StringBuffer str_b = new StringBuffer(b);
        results = new StringBuffer[b_len];
        for (int i = b_len - 1, k = 0; i >= 0; i--, k++) {
            results[k] = new StringBuffer(a_len + b_len);
            int digit_b = str_b.charAt(i) - '0';
            int jinwei = 0;
            int yushu = 0;
            for (int j = a_len - 1; j >= 0; j--) {
                int digit_a = str_a.charAt(j) - '0';
                int temp = digit_a * digit_b + jinwei;
                jinwei = temp / 10;
                yushu = temp % 10;
                results[k].append(yushu);
            }
            results[k].append(jinwei);
        }
        for (int i = 0; i < results.length; i++) {
            for (int j = 0; j < i; j++) {
                results[i].insert(0, '0');
            }
            for (int j = 0; j < results.length - 1 - i; j++) {
                results[i].append('0');
            }
        }
        System.out.println(add(results));
    }

    public static StringBuffer add(StringBuffer[] results) {
        StringBuffer stringBuffer = new StringBuffer();
        int nums = results.length;//加数的个数
        int jinwei = 0;
        for (int i = 0; i < results[0].length(); i++) {
            int tmp = 0;
            for (int j = 0; j < nums; j++) {
                tmp = tmp + results[j].charAt(i) - '0';
            }
            tmp += jinwei;
            jinwei = tmp / 10;
            int yushu = tmp % 10;
            stringBuffer.append(yushu);
        }
        stringBuffer.append(jinwei);
        stringBuffer = stringBuffer.reverse();
        while (stringBuffer.charAt(0) == '0') stringBuffer.deleteCharAt(0);
        return stringBuffer;
    }
}