链接：https://leetcode.com/problems/multiply-strings/
OJ好鬼畜啊
大数乘法板子题
结果需要转化为$string$型不然会$RE$
$Divide\;and\;conquer$(分治法)
将每一位算出来，最后进行进位和反转即可
//Time: 8ms Memory: 10.6MB
class Solution {
public:
string multiply(string num1, string num2) {
int len1 = num1.length(), len2 = num2.length(), len = 0;
int maxn = 110;
int a[maxn] = {0}, b[maxn] = {0}, c[2 * maxn] = {0};
char res[maxn * 2];
for(int j = 0, i = len1 - 1; i >= 0; --i) a[j++] = num1[i] - '0';
for(int j = 0, i = len2 - 1; i >= 0; --i) b[j++] = num2[i] - '0';
for(int i = 0; i < len2; ++i)
for(int j = 0; j < len1; ++j)
c[i + j] += a[j] * b[i];
for(int i = 0; i < 2 * maxn; ++i)
if(c[i] >= 10) c[i + 1] += c[i] / 10, c[i] %= 10;
for(int i = 0; i < 2 * maxn; ++i) res[i] = (char)(c[i] + '0');
for(int i = 2 * maxn - 1; i > 0; --i)
if(res[i] == '0') res[i] = '\0';
else break;
for(int i = 0; i < 2 * maxn; ++i)
if(res[i] != '\0') ++len;
else break;
reverse(res, res + len);
string ans(res);
return ans;
}
};