LeetCode 441 Arranging Coins

题目：

You have a total of n coins that you want to form in a staircase shape, where every k-th row must have exactly k coins.

Given n, find the total number of full staircase rows that can be formed.

n is a non-negative integer and fits within the range of a 32-bit signed integer.

Example 1:

n = 5The coins can form the following rows:¤¤ ¤¤ ¤Because the 3rd row is incomplete, we return 2.

Example 2:

n = 8The coins can form the following rows:¤¤ ¤¤ ¤ ¤¤ ¤Because the 4th row is incomplete, we return 3.

题目链接

题意：

给n个硬币，要求按要求排成多行，第k行需要k个硬币，返回可以按要求排成行的行数。

组成1行，需要1个

组成2行，需要1+2个

组成3行，需要1+2+3个

组成4行，需要1+2+3+4个

组成5行，需要1+2+3+4+5个。。。。。。。可推出公式

组成n行，需要（1+n）*n/2个

题目要求给出硬币，求出共有多少完整的行，就是对所推的公式求解，直接返回即可。

代码如下：

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