$i = 5$
Intially $5$ is printed, $i$ becomes $4$
if(4) is true, main() is called, $4$ is printed, $i$ becomes $3$
If(3) is true, main() is called, $3$ is printed, $i$ becomes $2$
if(2) is true, main() is called, $2$ is printed, $i $ becomes $1$
if(1) is true, main() is called, $1$ is printed, $i $ becomes $0$
If(0) is false, all function calls returns.
Hence 54321 is printed