Here, => means prints.
mystery3(0) => null
mystery3(1) => null
mystery3(2) => mystery3(0) 2 mystery3(1) => 2
mystery3(3) => mystery3(1) 3 mystery3(2) => 32
mystery3(4) => mystery3(2) 4 mystery3(3) => 2432
mystery3(5) => mystery3(3) 5 mystery3(4) => 3252432
mystery3(6) => mystery3(4) 6 mystery3(5) => 243263252432 (answer)