Difference between H(A)+H(B) and H(A+B) I was learning about Merkle trees and understood that if A, B, C and D are the leaf nodes, then their parents will be H(H(A)+H(B) and H(H(C)+H(D)) rsp. and the root will be H( H(H(A)+H(B)) + H(H(C)+H(D)) ). What if instead of this, the parents of the leaf node were: H(A+B) and H(C+D) and the root be H(A+B+C+D)?

I intuitively know this may not be a good choice but can someone explain it in a better way?
If + means concatenation, then $H(A+B+C+D)$ doesn't help make verification easier. You still need all leaves in order to verify any leaf, whereas the point of Merkle tree is to be able to verify individual leaves with only logarithmic cost.

If + means XOR or addition, then $H(A+B+C+D)$ is not collision-resistant so the construction fails to authenticate the leaves.