A tree is a hierarchical structure of nodes connected by edges, with no cycles. Each node holds a value and links to nodes below it. One node sits at the top as the root, the single entry point with no parent above it.
Parent and child describe a direct link: the node above is the parent, the nodes hanging off it are its children. A leaf is any node with no children, sitting at the bottom of a branch. Internal nodes have at least one child.
You reach every other node by following edges down from the root. Trees model anything with nested containment: file systems, HTML documents, org charts, and menus. The shape lets you narrow a search quickly, since each step down commits to one branch and drops the rest. That branching is why trees beat flat lists for hierarchical data.
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