阶乘之算法

function jieChengFdg(num) {
  var result = 1;
  if(num < 0) return -1;
  if(num == 0 || num == 1) return 1;
  while(num>1) {
    result *= num--;
  }
  return result;
}
console.log(jieChengFdg(5));//120

function jieChengDg(num) {
  var result = 1;
  if(num < 0) return -1;
  if(num == 0 || num == 1) return 1;
  if(num > 1) return num*jieChengDg(num-1);
}
console.log(jieChengDg(10));//3628800