Add Strings LeetCode solution in C++

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class Solution {
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 public:
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  string ans = "";
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  int carry = 0;
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  string addStrings(string num1, string num2) {
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    while (num1.size() && num2.size()) {
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      int sum = (num1.back() - '0' + num2.back() - '0' + carry);
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      ans = (char)((sum % 10) + '0') + ans;
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      carry = sum / 10;
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      num1.pop_back();
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      num2.pop_back();
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    }
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    while (num1.size()) {
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      int sum = (num1.back() - '0' + carry);
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      ans = (char)((sum % 10) + '0') + ans;
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      carry = sum / 10;
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      num1.pop_back();
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    }
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    while (num2.size()) {
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      int sum = (num2.back() - '0' + carry);
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      ans = (char)((sum % 10) + '0') + ans;
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      carry = sum / 10;
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      num2.pop_back();
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    }
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    if (carry) ans = (char)(carry + '0') + ans;
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    return ans;
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  }
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};