高精度 java_AcWing 793. JAVA: 高精度乘法 高精度*低精度 高精度*高精度

import java.io.*;

import java.util.*;

class Main{

//高精度*低精度

static ArrayList mulString(ArrayList a, int b){

ArrayList c = new ArrayList<>();

int res = 0;

for(int i = 0, t = 0; i < a.size(); ++i){

res += a.get(i) * b;

c.add(res % 10);

res /= 10;

}

if(res != 0)c.add(res);

while(c.size() > 1 && c.get(c.size() - 1) == 0)c.remove(c.size() - 1);

return c;

}

//高精度*高精度

static ArrayList mulStringHighLevel(ArrayList a, ArrayList b){

ArrayList c = new ArrayList<>(Collections.nCopies(a.size() + b.size(), 0));

for(int i = 0; i < a.size(); ++i)

for(int j = 0; j < b.size(); ++j)

c.set(i + j, c.get(i + j) + a.get(i) * b.get(j));

int res = 0;

for(int i = 0, t = 0; i < a.size(); ++i){

res += c.get(i);

c.set(i, res % 10);

res /= 10;

}

while(c.size() > 1 && c.get(c.size() - 1) == 0)c.remove(c.size() - 1);

return c;

}

public static void main(String[] args)throws Exception{

BufferedReader buf = new BufferedReader(new InputStreamReader(System.in));

String a = buf.readLine();

String b = buf.readLine();

ArrayList list = new ArrayList<>();

for(int i = a.length() - 1; i >= 0; --i)list.add(a.charAt(i) - '0');

int num = Integer.valueOf(b);

// ArrayList list1 = mulString(list, num);

ArrayList list1 = mulStringHighLevel(list, list);

for(int i = list1.size() - 1; i >= 0; --i){

System.out.print(list1.get(i));

}

}

}