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

I saw this solution in

Answer 1

Java implementation: (Based on @Ben Voigt)

BigInteger fact=1;
int sum=0;
int prod=1;
int x,y; // The 2 missing numbers
int n=a.length;
int max=MIN_VALUE;

for (int i=0; i