Create calculate-digit-sum-of-a-string.cpp

Author: kamyu104

C++/calculate-digit-sum-of-a-string.cpp

@@ -0,0 +1,23 @@
+// Time:  O(n + n * (log10(9k)/k) + ... + k)
+//      = O((n - (log10(9k)/k)*k)/(1-log10(9k)/k))
+//      = O(n / (1-log10(9k)/k)) = O(n) for k >= 2
+// Space: O(n)
+
+// simulation
+class Solution {
+public:
+    string digitSum(string s, int k) {
+        while (size(s) > k) {
+            string new_s;
+            for (int i = 0, curr = 0; i < size(s); ++i) {
+                curr += s[i] - '0';
+                if (i % k == k - 1 || i == size(s) - 1) {
+                    new_s += to_string(curr);
+                    curr = 0;
+                }
+            }
+            s = move(new_s);
+        }
+        return s;
+    }
+};