Update arranging-coins.cpp

Author: kamyu104

C++/arranging-coins.cpp

@@ -13,15 +13,20 @@ class Solution {
 class Solution2 {
 public:
     int arrangeCoins(int n) {
-        long long left = 1, right = n;
+        uint64_t left = 1, right = n;
         while (left <= right) {
             const auto mid = left + (right - left) / 2;
-            if (2L * n < mid * (mid + 1)) {
+            if (!check(mid, n)) {
                 right = mid - 1;
             } else {
                 left = mid + 1;
             }
         }
-        return left - 1;
+        return right;
+    }
+
+private:
+    bool check(uint64_t mid, uint64_t n) {
+        return mid <= 2L * n / (mid + 1);
     }
 };