String odd even character 2

#1

i didnot understand this question.can anyone explain???

https://hack.codingblocks.com/contests/c/509/238

#2

replace the character at odd position by its next character(a by b, b by c, …) and replace character at even position by its previous character(b by a, c by b,…).

My code for reference:
https://ide.codingblocks.com/#/s/28542