Tail Recusive

1.尾递归

double f(double guess){
　　if (isGoodEnough(guess)) return guess;
　　else return f(improve(guess));

}

2.优化

double f(double initialGuess){

　　double guess = initialGuess;

      while(!isGoodEnough(guess))

　　　　guess = improve(guess)

　　return guess;

}