C++ 大数运算（加减乘除取模）

 1 string add(string s1, string s2) {
 2     int len1 = s1.length(), len2 = s2.length();
 3     int maxlen = max(len1, len2) + 1;
 4     string res(maxlen, ‘0‘);
 5     int flag = 0;
 6     int i = len1 - 1, j = len2 - 1, k = maxlen - 1;
 7     for (; i >= 0 && j >= 0; i--, j--) {
 8         int temp = (s1[i] - ‘0‘) + (s2[j] - ‘0‘) + flag;
 9         flag = temp / 10;
10         res[k--]= temp % 10 + ‘0‘;
11     }
12     while (i >= 0) {
13         int temp = s1[i--] - ‘0‘ + flag;
14         flag = temp / 10;
15         res[k--] = temp % 10 + ‘0‘;
16     }
17     while (j >= 0) {
18         int temp = s2[j--] - ‘0‘ + flag;
19         flag = temp / 10;
20         res[k--] = temp % 10 + ‘0‘;
21     }
22     res[k] = flag + ‘0‘;
23     if (flag) {
24         return res;
25     } else {
26         return res.substr(1);
27     }
28 }