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  1. Recursive Digit Sum
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Recursive Digit Sum

  • dotaikhoa
     + 0 comments

    c++ solution

    because the input is large, we gotta avoid creating the initial string by concatenating string n k times. Instead we calculate the sum of initial n and then multiply it by k to get sum of digits of concatenated k times string n. From then on, we simulate the process in the description - get the sum of string until its length is one.

     #define ll long long

int superDigit(string n, int k) {
    string tmp = n;
    
    int len = tmp.size();
    int res = 0;
    
    while (len > 1) {
        ll sum = 0ll; 
        
        for (int i = 0; i < len; i++) {
            sum += int(tmp[i]-'0');
        }
        sum *= k;
        k=1;
        
        res = sum;
        
        string next = "";
        while (sum) {
            next += char((sum%10) + '0');
            sum /= 10;
        }
        tmp = string(next.rbegin(), next.rend());
        
        len = next.size();
    }
    return res;
}