Level order with a queue, step by step
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step | action                            | queue (front..back) | output so far
1    | poll 1, visit, offer its children | [2 3]               | [1]
2    | poll 2, visit, offer its children | [3 4 5]             | [1 2]
3    | poll 3, visit, offer its children | [4 5 6]             | [1 2 3]
4    | poll 4, visit, offer its children | [5 6]               | [1 2 3 4]
5    | poll 5, visit, offer its children | [6 7]               | [1 2 3 4 5]
6    | poll 6, visit, offer its children | [7 8 9]             | [1 2 3 4 5 6]
7    | poll 7, visit, offer its children | [8 9]               | [1 2 3 4 5 6 7]
8    | poll 8, visit, offer its children | [9]                 | [1 2 3 4 5 6 7 8]
9    | poll 9, visit, offer its children | []                  | [1 2 3 4 5 6 7 8 9]