✅ [C] || 100% Fast Solution || Beginner-Friendly
\nchar * digitSum(char * s, int k) {\n    int cur = 0;\n    int sum = 0;\n    for (int i = 0; strlen(s) > k; i++) {\n        if (i != 0 && i % k == 0 || i == st
```\nchar * digitSum(char * s, int k) {\n    int cur = 0;\n    int sum = 0;\n    for (int i = 0; strlen(s) > k; i++) {\n        if (i != 0 && i % k == 0 || i == strlen(s)) {\n                char buff[16];\n                sprintf(buff, "%d", sum);\n                for (int i = 0; buff[i]; i++, cur++)\n                    s[cur] = buff[i];\n                sum = 0;\n            }\n        if (i == strlen(s)) {    \n            s[cur] = \'\\0\';\n            cur = 0;\n            i = 0;\n        }\n        sum += s[i] - \'0\';\n    }\n    \n    return s;\n}\n```\n\n**If this was helpful, don\'t hesitate to upvote! :)**\nHave a nice day!
['C']