Post order traversal:

The children of node are visited before the node itself; the root is visited last. Each node is visited after its descendents are visited.

Algorithm for postorder traversal is following:

1. Traverse left sub-tree in post order

2. Traverse right sub-tree in post order

3. visit root node.

Determining the space occupied through files & directories in a file system need a postorder traversal as the space occupied through directory needs calculation of space needed by all files in the directory (children in tree structure)

          Figure:  Calculation of space occupied by a file system: A post order traversal

Since each of the nodes is traversed only once, the time complexity of post order traversal is T(n) = O(n), where n refer to number of nodes in the tree.