SOURCE

// Input:
// [4,3,2,7,8,2,3,1]

// Output:
// [5,6]
/**
 * @param {number[]} nums
 * @return {number[]}
 */
var findDisappearedNumbers = function(nums) {
    var n=nums.length;
  	var a={};
  var s=[];
  for(var i=0;i<n;i++){
    a[nums[i]]=i;
  }
  for(var i=1;i<=n;i++){
    //console.log(i,a[i])
    if(a[i]==undefined){
      s.push(i)
    }
  }
  return s;
};
console.log(findDisappearedNumbers([4,3,2,7,8,2,3,1]));