洗牌算法

1. 换牌法 ```javascript function swap(arr) { var len = arr.length

   for(var i = 0; i < length; i++){ var band = Math.floor(Math.random() * len) [arr[i], arr[band]] = [arr[band], arr[i]] } return arr }

swap([1,3,45,67,9])

2. 抽牌法
```javascript
function swap(arr){
    var len = arr.length
  var newArr = new Array()
  for(var i = len; i > 0; i--){
      var rand = Math.floor(Math.random()*len)
    newArr.push(arr[rand])
    arr[rand] = arr[i-1]
  }
  return newArr
}