Given two numbers represented as strings, return multiplication of the numbers as a string. Note: The numbers can be arbitrarily large and are non-negative. http://leetcode.com/onlinejudge#question_43

leetcode_multiply_strings.cpp

#define MAXN 10000
int f[MAXN];

class Solution {
public:
    string multiply(string num1, string num2) {
        int n = num1.length();
        int m = num2.length();
        memset(f, 0, sizeof(f));
        for (int i = m - 1; i >= 0; --i) {
            int st = m - 1 - i;
            for (int j = n - 1; j >= 0; --j) {
                f[st++] += (num2[i] - '0') * (num1[j] - '0');
            }
        }
        for (int i = 0; i < n + m; ++i) {
            f[i + 1] += f[i] / 10;
            f[i] %= 10;
        }
        int len;
        for (len = n + m - 1; len >= 0; --len)
            if (f[len] != 0)
                break;
        len = max(len, 0);
        string ans;
        for (int i = len; i >= 0; --i) {
            char c = '0' + f[i];
            ans += c;
        }
        return ans;
    }
};