Calculate Digit Sum of a String LeetCode solution in C++

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class Solution {
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 public:
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  string digitSum(string s, int k) {
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    if (s.length() <= k) return s;
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    string ans = "";
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    int sum = 0, temp = k;
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    int len = s.length();
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    for (int i = 0; i < len; i++) {
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      sum += (s[i] - '0');
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      temp--;
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      if (temp == 0) {
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        ans += to_string(sum);
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        temp = k;
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        sum = 0;
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      }
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    }
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    if (temp != k) {
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      ans += to_string(sum);
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    }
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    if (ans.length() > k) ans = digitSum(ans, k);
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    return ans;
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  }
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};