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