第一次写的复杂版：

string complexNumberMultiply(string a, string b) {
    int a1=0 ,b1=0 ,a2=0 ,b2=0;
    for (int i = 0; i<a.length(); i++) {
        int log = 0;
        if (a[i]=='+') {
            log = i;
            string tmp;
            for (int j=0; j<i; j++) {
                tmp+=a[j];
            }
            sscanf(tmp.c_str(), "%d",&a1);
            tmp = "";
            for (int m = i+1; m<a.length()-1; m++) {
                tmp+=a[m];
            }
            sscanf(tmp.c_str(), "%d",&b1);
        }
    }
    for (int i = 0; i<b.length(); i++) {
        int log = 0;
        if (b[i]=='+') {
            log = i;
            string tmp;
            for (int j=0; j<i; j++) {
                tmp+=b[j];
            }
            sscanf(tmp.c_str(), "%d",&a2);
            tmp = "";
            for (int m = i+1; m<b.length()-1; m++) {
                tmp+=b[m];
            }
            sscanf(tmp.c_str(), "%d",&b2);
        }
    }
    string res = to_string(a1*a2-b1*b2)+"+"+to_string(a1*b2+a2*b1)+"i";
    return res;
}

精简版：

class Solution {
public:
    string complexNumberMultiply(string a, string b) {
        int c,d,e,f;
        char ret[100];
        sscanf(a.c_str(),"%d+%di",&c,&d);
        sscanf(b.c_str(),"%d+%di",&e,&f);
        cout<<c<<" "<<d<<" "<<e<<" "<<f<<endl;
        sprintf(ret,"%d+%di",(c*e-f*d),(d*e+c*f));
        string ans(ret);
        return ans;
    }
};