sqrt算法的实现

sqrt()算法的实现：牛顿迭代法,求a的平方根

Xn+1=(Xn+a/Xn)/2;

还有一种方法，直接暴力得到最接近平方根的数，不用那么多习迭代。

float InvSqrt(float x)
{
    float xhalf = 0.5f*x;
    int i = *(int*)&x; // get bits for floating VALUE 
    i = 0x5f375a86- (i>>1); // gives initial guess y0
    x = *(float*)&i; // convert bits BACK to float
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy
    x = x*(1.5f-xhalf*x*x); // Newton step, repeating increases accuracy

    return 1/x;
}