http://mirror.codeforces.com/contest/1074/submission/45462396

http://mirror.codeforces.com/contest/1074/submission/45462405

These two codes are exactly the same, except for this part:

In the Wrong answer on test 14 submission:

for (int i : inTree[rootp]) {
	inTree[rootq].pb(i);
	distToRoot[i] = distToRoot[i] ^ distToRoot[p] ^ edgeWeight ^ distToRoot[q];
	root[i] = rootq;
}

In the Accepted submission:

edgeWeight ^= distToRoot[p] ^ distToRoot[q];

for (int i : inTree[rootp]) {
	inTree[rootq].pb(i);
	distToRoot[i] ^= edgeWeight;
	root[i] = rootq;
}

(This is the end of the method, so xor-equals edgeWeight shouldn't change anything later on.)

I thought xor sum is both commutative and associative? What's the difference between these two snippets? Am I missing something really obvious? Thanks in advance.