javascript 递归之阶乘

阶乘，即5! = 5*4*3*2*1, 先看传统的做法，利用while循环实现：

function factorial(num){
  var result = num;
  if(num<0){
    return -1;    //负数返回-1
  }
  if(num == 0){
    return 1;    // 0阶乘为1
  }
  
  while(num-- >2){

    result = result*num;
    
  }
  return result;
}

　javascript递归函数通过它调用其本身，利用递归的思路的话，阶乘的思路应该是 factorial(num)*factorial(num-1)*factorial(num-2) … factorial(0),即每次把num-1后作为参数调用自身：

function factorial2(num){
  if(num <0){
    return -1;
  }
  if(num == 0){
    return 1;
  }
  return num*(factorial2(num-1));
}