### LeetCode July Challenge - Day 1

**Problem:** LeetCode July Challenge - Day 1

**Ideas**

$$ \sum_{i=1}^k i = \frac{k\cdot (k+1)}{2}. $$

**Solution**:

```
class Solution {
public:
int arrangeCoins(int n) {
return (-1 + sqrt(1+8*(long long)n))/2;
}
};
```