Given two strings representing two complex numbers.

You need to return a string representing their multiplication. Note i2 = -1 according to the definition.

Example 1:

Input: "1+1i", "1+1i"
Output: "0+2i"
Explanation: (1 + i) * (1 + i) = 1 + i2 + 2 * i = 2i, and you need convert it to the form of 0+2i.
 

Example 2:

Input: "1+-1i", "1+-1i"
Output: "0+-2i"
Explanation: (1 - i) * (1 - i) = 1 + i2 - 2 * i = -2i, and you need convert it to the form of 0+-2i.

Note:

1. The input strings will not have extra blank.
2. The input strings will be given in the form of a+bi, where the integer a and b will both belong to the range of [-100, 100]. And the output should be also in this form.

Runtime: 4 ms, faster than 14.90% of C++ online submissions for Complex Number Multiplication.

class Solution {
public:
  pair<int,int> get_ab(string s){
    int addpos = 0;
    for(int i=0; i<s.size(); i++){
      if
 
(s[i] == '+') addpos = i;
    }
    auto p = make_pair<int,int>(0,0);
    p.first = stoi(s.substr(0,addpos));
    p.second = stoi(s.substr(addpos+1,s.size()-addpos-2));
    return p;
  }
  string complexNumberMultiply(string a, string b) {
    auto a_ab = get_ab(a);
    auto b_ab = get_ab(b);
    int x1 = a_ab.first, y1 = a_ab.second, x2 = b_ab.first, y2 = b_ab.second;
    string reta = to_string(x1*x2 - y1*y2);
    string retb = to_string(x1*y2 + x2*y1);

    return reta + "+" + retb + "i";
  }
};