441. Arranging Coins

https://leetcode.com/problems/arranging-coins/description/
Solution 1. Subtract the s-th row from n each time.
    int arrangeCoins(int n) {
        int s = 0;
        while(n>s) {
            s++;
            n -= s;
        }
        return s;
    }
Solution 2. A full staircase with s rows and n coins has s(s+1) = 2*n.
Thus s = (-1 + sqrt(1+8*n)) / 2 = (-0.25 + sqrt(2*n + 0.25).
    int arrangeCoins(int n) {
        return floor(-0.5+sqrt((double)2*n+0.25));
    }