leetcode 69. Sqrt(x)

int sqrt(int x)

int mySqrt(int x)
{
if (x == 0)
return 0;
long long int xx = x;
long long int up = xx;
long long int down = 0;
while (true)
{
if (up*up > xx)
up = (up+down)/2;
else
{
int t = down;
down = up;
up = 2 * up - t;
if (up-down==1&&up*up>=xx||up==down)
return up*up == x ? up : down;
}
}
}