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Let's try an example with n=4:

1 ^ 2 ^ 3 ^ 4 ^ A[0] ^ A[1] ^ A[2] ^ A[3]

It might be early in the morning and I am missing something, but it has n-1 as its last index. Any insight is appreciated :)

Edit: There is a missing number in the array, thus its last index is n-2. Thanks for the correction, OP.

Premise: A is containing all values in range [1, 4] except one:

Let's say that, without loss of generality:

> 1 ^ 2 ^ 3 ^ 4 ^ A[0]=1 ^ A[1]=2 ^ A[2]=3 ^ A[3]=4

However, A is now missing no value. Contradiction.

∎

But there are only 3 items in the array (for example 1, 2 and 4). So the last valid index is 2.

Ah. I was about to make an edit to my comment as you are correct :) Thanks!