树：节点的深度与高度定义

（考虑深度时，把树看作液体，节点越往下越深；考虑高度时，把树当作建筑，从最深的子孙-地基数上来。节点和兄弟，深度一定相同，高度不一定相同。）

origin:https://stackoverflow.com/a/2603707/2098804

I learned that depth and height are properties of a node:

• The depth of a node is the number of edges from the node to the tree’s root node.
  A root node will have a depth of 0.
• The height of a node is the number of edges on the longest path from the node to a leaf.
  A leaf node will have a height of 0.

Properties of a tree:

• The height of a tree would be the height of its root node,
  or equivalently, the depth of its deepest node.
• The diameter (or width) of a tree is the number of nodes on the longest path between any two leaf nodes. The tree below has a diameter of 6 nodes.