牛客2018校招 1. 拼多多 大整數相乘

解題的思路並不複雜：模擬算數的乘法運算過程
可是一次直接AC仍是比較困難的。
注意點：
1.進位 每一輪的進位最終存在的話，還須要加上
2.高位和低位的存儲問題
3.輸入，輸出問題
輸出ACM，採用System,out.println();

import java.io.*;
import java.util.*;

public class Main{
    public static String mul(String s1, String s2){
       int[] i1 = new int[s1.length()];
       int[] i2 = new int[s2.length()];
       int[] res = new int[s1.length() + s2.length()];
       for (int i = 0; i < s1.length(); i++)
           i1[i] = s1.charAt(i) - '0';
       for (int i = 0;i < s2.length() ;i++)
           i2[i] = s2.charAt(i) - '0';
       
       int ji = 0;
       int k = i2.length;
       int i = 0, j = 0;
       for (i = i2.length - 1;i >= 0 ; i--){
           for(j = i1.length - 1; j >= 0; j--){
               res[k + j] += (i2[i]* i1[j]) % 10 + ji;
               ji = (i2[i]* i1[j]) / 10;
               if (res[k + j] >= 10){
                   res[k + j - 1] += res[k + j] / 10;
                   res[k + j] = res[k + j] % 10;
               }
           }
           if (ji > 0)    res[k + j] += ji;
           ji = 0;
           k --;
       }
        
       boolean b = false;
       String sr = "";
       for (i = 0; i < res.length; i ++)
           if (b)
               sr += String.valueOf(res[i]);
           else
               if (res[i] != 0){
                   b = true;
                   sr += String.valueOf(res[i]);
               }
       return sr;
    }

    public static void main(String args[]) throws IOException{
        BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
        String[] input = br.readLine().split(" ");
        String res = mul(input[0], input[1]);
        System.out.println(res);
    }
}