There is error in timing, it is written "Time: 8 PM — 10:30 PM IST", but contest page says for 3 hours.

Why is this not rated for Div-2 on Codechef?

Majority of audience lies in Div-2 on Codechef !

I have a funny, non-constructive solution for nonzero subarray xor.

Suppose we already constructed some prefix $$$a_1, ..., a_i$$$. Each suffix of this prefix "bans" at most one value of $$$a_{i+1}$$$, for a total of $$$i \le 2 \cdot 10^5$$$ banned values. However, there are $$$10^6$$$ possible choices for $$$a_{i+1}$$$. It follows, that if we take a random candidate for $$$a_{i+1}$$$ then it will work with a probability of $$$\ge 1 - 2 \cdot 10^5 / 10^6 = 4/5$$$. Hence we will make an expected $$$O(n)$$$ tries overall, which is fast enough if we can check if a candidate is good quickly.

This transforms a problem into keeping a set of suffix xors, which is well-known. Implementation. Essentially each suffix xor is a xor of the complement prefix xor and the xor of the entire array, hence we only need to keep track of xor of the entire array.