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

I have an algorithm for above problem.

Suppose

Actual Series: 1 2 3 4 5 6          a:sum=21 product= 720
Missing Number series: 1 * 3 4 * 6  b:sum=14 product= 72

a+b=21-14= 7 , ab=720/72=10

Now we need to find a-b= sqrt[(a+b)^2 -4ab].

If you calculate:

a-b= 3

Now

a+b=7
a-b=3

Add both equations:

2a=10, a=5

then b=7-5=2 so, 2 and 5 are missing.