Integer to English Words LeetCode solution in C++

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unordered_map<int, string> mp_0 = {
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    {1, "One "},      {2, "Two "},        {3, "Three "},     {4, "Four "},      {5, "Five "},
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    {6, "Six "},      {7, "Seven "},      {8, "Eight "},     {9, "Nine "},      {10, "Ten "},
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    {11, "Eleven "},  {12, "Twelve "},    {13, "Thirteen "}, {14, "Fourteen "}, {15, "Fifteen "},
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    {16, "Sixteen "}, {17, "Seventeen "}, {18, "Eighteen "}, {19, "Nineteen "}};
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unordered_map<int, string> mp_10 = {{20, "Twenty "}, {30, "Thirty "}, {40, "Forty "},
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                                    {50, "Fifty "},  {60, "Sixty "},  {70, "Seventy "},
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                                    {80, "Eighty "}, {90, "Ninety "}};
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class Solution {
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  void Solve2(string &ans, int num) {
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    ans = mp_0[num] + ans;
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  }
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  void Solve(string &ans, int num) {
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    int factor = 1;
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    for (int i = 0; num > 0; i++) {
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      if (factor == 1) {
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        if (num % 100 < 20) {
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          Solve2(ans, num % 100);
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          factor = factor * 10;
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          num = num / 10;
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        } else {
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          if (num % 10 != 0) ans = mp_0[num % 10] + ans;
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        }
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      }
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      else if (factor == 10) {
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        if (num % 10 != 0) ans = mp_10[(num % 10) * 10] + ans;
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      }
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      else if (factor == 100) {
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        if (num % 10 != 0) ans = (mp_0[num % 10] + "Hundred ") + ans;
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      }
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      factor = factor * 10;
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      num = num / 10;
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    }
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  }
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 public:
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  string numberToWords(int num) {
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    // 2,147,483,647
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    if (num == 0) return "Zero";
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    string ans;
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    for (int i = 0; num > 0; i++) {
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      if (i == 1) {
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        if (num % 1000 > 0) ans = "Thousand " + ans;
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      } else if (i == 2) {
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        if (num % 1000 > 0) ans = "Million " + ans;
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      } else if (i == 3)
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        ans = "Billion " + ans;
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      Solve(ans, num % 1000);
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      num = num / 1000;
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    }
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    ans.pop_back();
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    return ans;
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  }
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};