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 = 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.
链接：https://leetcode.com/problems/arranging-coins/#/description
3/25/2017
performance 4%, 87ms
错误：
1. 输入最大值
2. i的最大范围
1 public class Solution { 2 public int arrangeCoins(int n) { 3 int m = n; 4 for (int i = 0; i <= m; i++) { 5 n -= i; 6 if (n == 0) return i; 7 else if (n < 0) return i - 1; 8 } 9 return -1; 10 } 11 }
别人的方法：
binary search
算式
https://discuss.leetcode.com/topic/65575/java-o-1-solution-math-problem/5
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