441.Arranging Coins

难度:Easy

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 = 5
​
The coins can form the following rows:
¤
¤ ¤
¤ ¤
​
Because the 3rd row is incomplete, we return 2.
Example 2:
​
n = 8
​
The coins can form the following rows:
¤
¤ ¤
¤ ¤ ¤
¤ ¤
​
Because the 4th row is incomplete, we return 3.

简单的等差数列求项数的题目，一元二次方程求根公式。

class Solution {
public:
    int arrangeCoins(int n) {
        return int((sqrt(1+8.0*n)-1)/2);
    }
};

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