Find 2 missing numbers in an array of integers with two missing values

Question

How do you do this? The values are unsorted but are of [1..n] Example array [3,1,2,5,7,8]. Answer: 4, 6

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Answer 1

Below is the generic answer in java code for any number of missing numbers in a given array
//assumes that there are no duplicates
a = [1,2,3,4,5]
b = [1,2,5]
a-b=[3,4]

public list find(int[] input){
  int[] a= new int[] {1,2,3,4,5};//create a new array without missing numbers
  List l = new ArrayList();//list for missing numbers
  Map m = new HashMap();

  //populate a hashmap with the elements in the new array
  for(int i=0;i