Actually, I think this problem needs more explanations.
X is not the perfect square number, besides, we want to find the closest int which is smaller than or equals to exact sqrt(x).
More explanations for the problem

@liushu1231 said in More explanations for the problem:
The problem's type of the return value is int, so it's clear to find the closet int which is smaller than or equals to exact sqrt(x).
IT IS NOT CLEAR AT ALL!
SQRT(15) is what? 3.87... something. Should I round it up or just strip off to 3?