Merkle Tree
this is a REALLY cool topic. A Merkle tree is a tree in which every “leaf” (node) is labelled with the Cryptographic Hash of a data block, and every node that is not a leaf (called a branch, inner node, or inode) is labelled with the cryptographic hash of the labels of its child nodes
Applications
Detecting changes in a file. Git does that, at the top first. If so, check bottom two.