進数変換

1
string res = convNotation(str,n,m);

1
string convNotation(const string &s, int from, int to) {
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  int n = stoll(s, nullptr, from);
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  if (n == 0)
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    return "0";
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  string result;
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  while (n) {
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    result += n % to < 10 ? '0' + n % to : 'A' + n % to - 10;
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    n /= to;
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  }
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  if (s[0] == '-')
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    result += '-';
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  reverse(result.begin(), result.end());
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  return result;
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}