441. Arranging Coins

题目来源【Leetcode】

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:
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¤ ¤
¤ ¤

Because the 3rd row is incomplete, we return 2.
Example 2:

n = 8

The coins can form the following rows:
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¤ ¤
¤ ¤ ¤
¤ ¤

Because the 4th row is incomplete, we return 3.

这道题用一个公式：（i+1）*i/2 >= n来解，不需要循环；

class Solution {
public:
    int arrangeCoins(int n) {
        double i = (double)sqrt((long long)2*n+0.25)-0.5;
        return (int)i;
    }
};