Complex Number Multiplication LeetCode solution in C++

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class Solution {
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 public:
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  pair<int, int> parse(string num) {
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    int i = num.find('+');
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    double real = stoi(num.substr(0, i));
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    double imaginary = stoi(num.substr(i + 1, num.size() - i - 2));
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    pair<int, int> res(real, imaginary);
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    return res;
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  }
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  string complexNumberMultiply(string num1, string num2) {
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    pair<int, int> a = parse(num1), b = parse(num2);
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    int real_a = a.first, imag_a = a.second;
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    int real_b = b.first, imag_b = b.second;
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    return to_string(real_a * real_b - imag_a * imag_b) + '+' +
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           to_string(real_a * imag_b + real_b * imag_a) + 'i';
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  }
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};