题目描述：
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.
其实就是求小于N的最大M，M为1+...+K。
思路：
遍历1到n/2即可。
实现代码：
public int ArrangeCoins(int n) {
if(n == 0){
return 0;
}
if(n == 1){
return 1;
}
var half = n/2 + 1;
var sum = 0;
var count = 0;
for(var i = 1;sum >= 0 && i <= half && sum < n; i++){
sum += i;
count ++;
}
if(sum > n || sum < 0){
count --;
}
//Console.WriteLine(sum);
return count;
}