模板code
考虑mid vs x
如果mid <= x，答案在[mid, r]之间；
如果mid*mid > x, 答案在[l, mid - 1]之间；

第二种分法，mid = (l + r + 1)//2

class Solution:
    def mySqrt(self, x: int) -> int:
        if x == 0 or x == 1 : return x
        l, r = 0, x
        while l < r:
            mid = (l + r + 1)//2          #//整数除法
            if mid <= x//mid:
                l = mid
            else:
                r = mid - 1
        return l