Iterative postorder with one stack, step by step
================================================

step | action                                   | stack (bottom..top) | output so far
1    | push 1                                   | [1]                 | []
2    | push 2                                   | [1 2]               | []
3    | push 4                                   | [1 2 4]             | []
4    | pop 4, visit                             | [1 2]               | [4]
5    | peek 2: right child 5 not done, go right | [1 2]               | [4]
6    | push 5                                   | [1 2 5]             | [4]
7    | push 7                                   | [1 2 5 7]           | [4]
8    | pop 7, visit                             | [1 2 5]             | [4 7]
9    | pop 5, visit                             | [1 2]               | [4 7 5]
10   | pop 2, visit                             | [1]                 | [4 7 5 2]
11   | peek 1: right child 3 not done, go right | [1]                 | [4 7 5 2]
12   | push 3                                   | [1 3]               | [4 7 5 2]
13   | peek 3: right child 6 not done, go right | [1 3]               | [4 7 5 2]
14   | push 6                                   | [1 3 6]             | [4 7 5 2]
15   | push 8                                   | [1 3 6 8]           | [4 7 5 2]
16   | pop 8, visit                             | [1 3 6]             | [4 7 5 2 8]
17   | peek 6: right child 9 not done, go right | [1 3 6]             | [4 7 5 2 8]
18   | push 9                                   | [1 3 6 9]           | [4 7 5 2 8]
19   | pop 9, visit                             | [1 3 6]             | [4 7 5 2 8 9]
20   | pop 6, visit                             | [1 3]               | [4 7 5 2 8 9 6]
21   | pop 3, visit                             | [1]                 | [4 7 5 2 8 9 6 3]
22   | pop 1, visit                             | []                  | [4 7 5 2 8 9 6 3 1]