441_ Arranging_Coins

/*441. Arranging CoinsYou 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.*///思路，就像高斯定理一样，首项加末项乘香树除以2就是n//解法一  83%  23msint arrangeCoins(int n) {    long long int i = n;    i = (sqrt(8*i+1)-1)/2;    return i;}/解法二   98%  19ms//我试着把long long int 改成float，但是2146467959数据没法通过， 超出了范围，如果改成double就可以了。直接怼到98.65%int arrangeCoins(int n) {    double i = n;    i = (sqrt(8*i+1)-1)*0.5;    return i;}