441. Arranging Coins

 static int wing=[]()
 {
     std::ios::sync_with_stdio(false);
     cin.tie(NULL);
     return ;
 }();

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

直接数学公式解题