找出数组 arr 中重复出现过的元素（不用考虑返回顺序）

var findRepeat = (arr) =>{
  let map = new Map()
  let res = []
  for(let num of arr){
    map.set(num, (map.get(num) || 0)+1)
  }
  map.forEach((val, key)=>{
    if(val > 1) {
      res.push(key)
    }
  })
  return res
}
console.log(arr, findRepeat(arr))
var findRepeat = (arr) =>{
  arr.sort((a, b)=> a - b)
  let n = arr.length
  let newArr = new Set()
  for(let i = 1; i < n ; i++){
    if(arr[i] == arr[i - 1]){
      newArr.add(arr[i])
    }
  }
  return new Array(...newArr)
}

console.log(arr, findRepeat(arr))