Binary Search:441. Arranging Coins

这道题给了我们n个硬币，让我们按一定规律排列，第一行放1个，第二行放2个，以此类推，问我们有多少行能放满。

注意low，high相加可能大于整数范围，所以要用long

class Solution {public:    int arrangeCoins(int n) {        if (n <= 1) return n;        long low = 1, high = n;        while (low < high) {            long mid = low + (high - low) / 2;            if (mid * (mid + 1) / 2 <= n) low = mid + 1;            else high = mid;        }        return low - 1;    }};