Multiply Strings

multiply-strings.c

char* multiply(char* num1, char* num2) {
    if (!num1 || !num2) return NULL;
    if (num1[0] == '0' || num2[0] == '0') return "0";
    int len1 = strlen(num1), len2 = strlen(num2);
    if (len1 == 1 && num1[0] == '1') return num2;
    if (len2 == 1 && num2[0] == '1') return num1;
    char *result = malloc((len1+len2)*sizeof(char));
    memset(result, '0', (len1+len2)*sizeof(char));
    for (int i = len1-1; i >= 0; i--) {
        int carry = 0;
        for (int j = len2-1; j >= 0; j--) {
            int sum = (result[i+j+1] - '0') + (num1[i] - '0') * (num2[j] - '0') + carry;
            result[i+j+1] = sum % 10 + '0';
            carry = sum / 10;
        }
        result[i] += carry;
    }
    int i = 0;
    while (result[i] == '0') i++;
    return &result[i];
}