题目连接：Leetcode 043 Multiply Strings
解题思路：裸的大数乘法。
class Solution { public: string reverseString(string s) { int n = s.size(); string ret = s; for (int i = 0; i < n; i++) ret[i] = s[n-i-1]; return ret; } string multiply(string num1, string num2) { num1 = reverseString(num1); num2 = reverseString(num2); int t; string ans = ""; for (int i = 0; i < num1.size(); i++) { t = 0; for (int j = 0; j < num2.size(); j++) { if (i+j >= ans.size()) ans += '0'; t += (num1[i]-'0') * (num2[j]-'0') + (ans[i+j]-'0'); ans[i+j] = '0' + t % 10; t /= 10; } int k = i + num2.size(); while (t) { if (k >= ans.size()) ans += '0'; t += ans[k] - '0'; ans[k++] = '0' + t % 10; t /= 10; } } int n = ans.size(); while (n && ans[n-1] == '0') n--; ans = reverseString(ans.substr(0, max(n, 1))); return ans; } };